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+What is RCU?
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+
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+RCU is a synchronization mechanism that was added to the Linux kernel
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+during the 2.5 development effort that is optimized for read-mostly
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+situations. Although RCU is actually quite simple once you understand it,
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+getting there can sometimes be a challenge. Part of the problem is that
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+most of the past descriptions of RCU have been written with the mistaken
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+assumption that there is "one true way" to describe RCU. Instead,
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+the experience has been that different people must take different paths
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+to arrive at an understanding of RCU. This document provides several
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+different paths, as follows:
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+
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+1. RCU OVERVIEW
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+2. WHAT IS RCU'S CORE API?
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+3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
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+4. WHAT IF MY UPDATING THREAD CANNOT BLOCK?
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+5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
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+6. ANALOGY WITH READER-WRITER LOCKING
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+7. FULL LIST OF RCU APIs
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+8. ANSWERS TO QUICK QUIZZES
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+
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+People who prefer starting with a conceptual overview should focus on
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+Section 1, though most readers will profit by reading this section at
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+some point. People who prefer to start with an API that they can then
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+experiment with should focus on Section 2. People who prefer to start
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+with example uses should focus on Sections 3 and 4. People who need to
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+understand the RCU implementation should focus on Section 5, then dive
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+into the kernel source code. People who reason best by analogy should
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+focus on Section 6. Section 7 serves as an index to the docbook API
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+documentation, and Section 8 is the traditional answer key.
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+
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+So, start with the section that makes the most sense to you and your
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+preferred method of learning. If you need to know everything about
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+everything, feel free to read the whole thing -- but if you are really
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+that type of person, you have perused the source code and will therefore
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+never need this document anyway. ;-)
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+
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+
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+1. RCU OVERVIEW
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+
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+The basic idea behind RCU is to split updates into "removal" and
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+"reclamation" phases. The removal phase removes references to data items
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+within a data structure (possibly by replacing them with references to
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+new versions of these data items), and can run concurrently with readers.
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+The reason that it is safe to run the removal phase concurrently with
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+readers is the semantics of modern CPUs guarantee that readers will see
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+either the old or the new version of the data structure rather than a
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+partially updated reference. The reclamation phase does the work of reclaiming
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+(e.g., freeing) the data items removed from the data structure during the
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+removal phase. Because reclaiming data items can disrupt any readers
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+concurrently referencing those data items, the reclamation phase must
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+not start until readers no longer hold references to those data items.
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+
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+Splitting the update into removal and reclamation phases permits the
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+updater to perform the removal phase immediately, and to defer the
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+reclamation phase until all readers active during the removal phase have
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+completed, either by blocking until they finish or by registering a
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+callback that is invoked after they finish. Only readers that are active
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+during the removal phase need be considered, because any reader starting
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+after the removal phase will be unable to gain a reference to the removed
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+data items, and therefore cannot be disrupted by the reclamation phase.
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+
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+So the typical RCU update sequence goes something like the following:
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+
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+a. Remove pointers to a data structure, so that subsequent
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+ readers cannot gain a reference to it.
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+
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+b. Wait for all previous readers to complete their RCU read-side
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+ critical sections.
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+
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+c. At this point, there cannot be any readers who hold references
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+ to the data structure, so it now may safely be reclaimed
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+ (e.g., kfree()d).
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+
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+Step (b) above is the key idea underlying RCU's deferred destruction.
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+The ability to wait until all readers are done allows RCU readers to
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+use much lighter-weight synchronization, in some cases, absolutely no
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+synchronization at all. In contrast, in more conventional lock-based
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+schemes, readers must use heavy-weight synchronization in order to
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+prevent an updater from deleting the data structure out from under them.
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+This is because lock-based updaters typically update data items in place,
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+and must therefore exclude readers. In contrast, RCU-based updaters
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+typically take advantage of the fact that writes to single aligned
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+pointers are atomic on modern CPUs, allowing atomic insertion, removal,
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+and replacement of data items in a linked structure without disrupting
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+readers. Concurrent RCU readers can then continue accessing the old
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+versions, and can dispense with the atomic operations, memory barriers,
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+and communications cache misses that are so expensive on present-day
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+SMP computer systems, even in absence of lock contention.
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+
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+In the three-step procedure shown above, the updater is performing both
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+the removal and the reclamation step, but it is often helpful for an
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+entirely different thread to do the reclamation, as is in fact the case
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+in the Linux kernel's directory-entry cache (dcache). Even if the same
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+thread performs both the update step (step (a) above) and the reclamation
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+step (step (c) above), it is often helpful to think of them separately.
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+For example, RCU readers and updaters need not communicate at all,
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+but RCU provides implicit low-overhead communication between readers
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+and reclaimers, namely, in step (b) above.
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+
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+So how the heck can a reclaimer tell when a reader is done, given
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+that readers are not doing any sort of synchronization operations???
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+Read on to learn about how RCU's API makes this easy.
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+
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+
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+2. WHAT IS RCU'S CORE API?
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+
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+The core RCU API is quite small:
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+
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+a. rcu_read_lock()
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+b. rcu_read_unlock()
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+c. synchronize_rcu() / call_rcu()
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+d. rcu_assign_pointer()
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+e. rcu_dereference()
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+
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+There are many other members of the RCU API, but the rest can be
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+expressed in terms of these five, though most implementations instead
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+express synchronize_rcu() in terms of the call_rcu() callback API.
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+
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+The five core RCU APIs are described below, the other 18 will be enumerated
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+later. See the kernel docbook documentation for more info, or look directly
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+at the function header comments.
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+
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+rcu_read_lock()
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+
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+ void rcu_read_lock(void);
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+
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+ Used by a reader to inform the reclaimer that the reader is
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+ entering an RCU read-side critical section. It is illegal
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+ to block while in an RCU read-side critical section, though
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+ kernels built with CONFIG_PREEMPT_RCU can preempt RCU read-side
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+ critical sections. Any RCU-protected data structure accessed
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+ during an RCU read-side critical section is guaranteed to remain
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+ unreclaimed for the full duration of that critical section.
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+ Reference counts may be used in conjunction with RCU to maintain
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+ longer-term references to data structures.
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+
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+rcu_read_unlock()
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+
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+ void rcu_read_unlock(void);
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+
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+ Used by a reader to inform the reclaimer that the reader is
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+ exiting an RCU read-side critical section. Note that RCU
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+ read-side critical sections may be nested and/or overlapping.
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+
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+synchronize_rcu()
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+
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+ void synchronize_rcu(void);
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+
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+ Marks the end of updater code and the beginning of reclaimer
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+ code. It does this by blocking until all pre-existing RCU
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+ read-side critical sections on all CPUs have completed.
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+ Note that synchronize_rcu() will -not- necessarily wait for
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+ any subsequent RCU read-side critical sections to complete.
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+ For example, consider the following sequence of events:
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+
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+ CPU 0 CPU 1 CPU 2
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+ ----------------- ------------------------- ---------------
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+ 1. rcu_read_lock()
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+ 2. enters synchronize_rcu()
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+ 3. rcu_read_lock()
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+ 4. rcu_read_unlock()
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+ 5. exits synchronize_rcu()
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+ 6. rcu_read_unlock()
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+
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+ To reiterate, synchronize_rcu() waits only for ongoing RCU
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+ read-side critical sections to complete, not necessarily for
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+ any that begin after synchronize_rcu() is invoked.
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+
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+ Of course, synchronize_rcu() does not necessarily return
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+ -immediately- after the last pre-existing RCU read-side critical
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+ section completes. For one thing, there might well be scheduling
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+ delays. For another thing, many RCU implementations process
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+ requests in batches in order to improve efficiencies, which can
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+ further delay synchronize_rcu().
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+
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+ Since synchronize_rcu() is the API that must figure out when
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+ readers are done, its implementation is key to RCU. For RCU
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+ to be useful in all but the most read-intensive situations,
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+ synchronize_rcu()'s overhead must also be quite small.
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+
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+ The call_rcu() API is a callback form of synchronize_rcu(),
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+ and is described in more detail in a later section. Instead of
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+ blocking, it registers a function and argument which are invoked
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+ after all ongoing RCU read-side critical sections have completed.
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+ This callback variant is particularly useful in situations where
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+ it is illegal to block.
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+
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+rcu_assign_pointer()
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+
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+ typeof(p) rcu_assign_pointer(p, typeof(p) v);
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+
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+ Yes, rcu_assign_pointer() -is- implemented as a macro, though it
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+ would be cool to be able to declare a function in this manner.
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+ (Compiler experts will no doubt disagree.)
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+
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+ The updater uses this function to assign a new value to an
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+ RCU-protected pointer, in order to safely communicate the change
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+ in value from the updater to the reader. This function returns
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+ the new value, and also executes any memory-barrier instructions
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+ required for a given CPU architecture.
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+
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+ Perhaps more important, it serves to document which pointers
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+ are protected by RCU. That said, rcu_assign_pointer() is most
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+ frequently used indirectly, via the _rcu list-manipulation
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+ primitives such as list_add_rcu().
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+
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+rcu_dereference()
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+
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+ typeof(p) rcu_dereference(p);
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+
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+ Like rcu_assign_pointer(), rcu_dereference() must be implemented
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+ as a macro.
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+
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+ The reader uses rcu_dereference() to fetch an RCU-protected
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+ pointer, which returns a value that may then be safely
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+ dereferenced. Note that rcu_deference() does not actually
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+ dereference the pointer, instead, it protects the pointer for
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+ later dereferencing. It also executes any needed memory-barrier
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+ instructions for a given CPU architecture. Currently, only Alpha
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+ needs memory barriers within rcu_dereference() -- on other CPUs,
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+ it compiles to nothing, not even a compiler directive.
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+
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+ Common coding practice uses rcu_dereference() to copy an
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+ RCU-protected pointer to a local variable, then dereferences
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+ this local variable, for example as follows:
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+
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+ p = rcu_dereference(head.next);
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+ return p->data;
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+
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+ However, in this case, one could just as easily combine these
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+ into one statement:
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+
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+ return rcu_dereference(head.next)->data;
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+
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+ If you are going to be fetching multiple fields from the
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+ RCU-protected structure, using the local variable is of
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+ course preferred. Repeated rcu_dereference() calls look
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+ ugly and incur unnecessary overhead on Alpha CPUs.
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+
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+ Note that the value returned by rcu_dereference() is valid
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+ only within the enclosing RCU read-side critical section.
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+ For example, the following is -not- legal:
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+
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+ rcu_read_lock();
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+ p = rcu_dereference(head.next);
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+ rcu_read_unlock();
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+ x = p->address;
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+ rcu_read_lock();
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+ y = p->data;
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+ rcu_read_unlock();
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+
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+ Holding a reference from one RCU read-side critical section
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+ to another is just as illegal as holding a reference from
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+ one lock-based critical section to another! Similarly,
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+ using a reference outside of the critical section in which
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+ it was acquired is just as illegal as doing so with normal
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+ locking.
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+
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+ As with rcu_assign_pointer(), an important function of
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+ rcu_dereference() is to document which pointers are protected
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+ by RCU. And, again like rcu_assign_pointer(), rcu_dereference()
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+ is typically used indirectly, via the _rcu list-manipulation
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+ primitives, such as list_for_each_entry_rcu().
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+
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+The following diagram shows how each API communicates among the
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+reader, updater, and reclaimer.
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+
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+
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+ rcu_assign_pointer()
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+ +--------+
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+ +---------------------->| reader |---------+
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+ | +--------+ |
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+ | | |
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+ | | | Protect:
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+ | | | rcu_read_lock()
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+ | | | rcu_read_unlock()
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+ | rcu_dereference() | |
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+ +---------+ | |
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+ | updater |<---------------------+ |
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+ +---------+ V
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+ | +-----------+
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+ +----------------------------------->| reclaimer |
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+ +-----------+
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+ Defer:
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+ synchronize_rcu() & call_rcu()
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+
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+
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+The RCU infrastructure observes the time sequence of rcu_read_lock(),
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+rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in
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+order to determine when (1) synchronize_rcu() invocations may return
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+to their callers and (2) call_rcu() callbacks may be invoked. Efficient
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+implementations of the RCU infrastructure make heavy use of batching in
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+order to amortize their overhead over many uses of the corresponding APIs.
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+
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+There are no fewer than three RCU mechanisms in the Linux kernel; the
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+diagram above shows the first one, which is by far the most commonly used.
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+The rcu_dereference() and rcu_assign_pointer() primitives are used for
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+all three mechanisms, but different defer and protect primitives are
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+used as follows:
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+
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+ Defer Protect
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+
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+a. synchronize_rcu() rcu_read_lock() / rcu_read_unlock()
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+ call_rcu()
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+
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+b. call_rcu_bh() rcu_read_lock_bh() / rcu_read_unlock_bh()
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+
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+c. synchronize_sched() preempt_disable() / preempt_enable()
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+ local_irq_save() / local_irq_restore()
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+ hardirq enter / hardirq exit
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+ NMI enter / NMI exit
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+
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+These three mechanisms are used as follows:
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+
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+a. RCU applied to normal data structures.
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+
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+b. RCU applied to networking data structures that may be subjected
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+ to remote denial-of-service attacks.
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+
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+c. RCU applied to scheduler and interrupt/NMI-handler tasks.
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+
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+Again, most uses will be of (a). The (b) and (c) cases are important
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+for specialized uses, but are relatively uncommon.
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+
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+
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+3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
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+
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+This section shows a simple use of the core RCU API to protect a
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+global pointer to a dynamically allocated structure. More typical
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+uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt.
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+
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+ struct foo {
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+ int a;
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+ char b;
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+ long c;
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+ };
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+ DEFINE_SPINLOCK(foo_mutex);
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+
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+ struct foo *gbl_foo;
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+
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+ /*
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+ * Create a new struct foo that is the same as the one currently
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+ * pointed to by gbl_foo, except that field "a" is replaced
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+ * with "new_a". Points gbl_foo to the new structure, and
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+ * frees up the old structure after a grace period.
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+ *
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+ * Uses rcu_assign_pointer() to ensure that concurrent readers
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+ * see the initialized version of the new structure.
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+ *
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+ * Uses synchronize_rcu() to ensure that any readers that might
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+ * have references to the old structure complete before freeing
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+ * the old structure.
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+ */
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+ void foo_update_a(int new_a)
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+ {
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+ struct foo *new_fp;
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+ struct foo *old_fp;
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+
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+ new_fp = kmalloc(sizeof(*fp), GFP_KERNEL);
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+ spin_lock(&foo_mutex);
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+ old_fp = gbl_foo;
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+ *new_fp = *old_fp;
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+ new_fp->a = new_a;
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+ rcu_assign_pointer(gbl_foo, new_fp);
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+ spin_unlock(&foo_mutex);
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+ synchronize_rcu();
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+ kfree(old_fp);
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+ }
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+
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+ /*
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+ * Return the value of field "a" of the current gbl_foo
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+ * structure. Use rcu_read_lock() and rcu_read_unlock()
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+ * to ensure that the structure does not get deleted out
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+ * from under us, and use rcu_dereference() to ensure that
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+ * we see the initialized version of the structure (important
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+ * for DEC Alpha and for people reading the code).
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+ */
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+ int foo_get_a(void)
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+ {
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+ int retval;
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+
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+ rcu_read_lock();
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+ retval = rcu_dereference(gbl_foo)->a;
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+ rcu_read_unlock();
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+ return retval;
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|
|
+ }
|
|
|
+
|
|
|
+So, to sum up:
|
|
|
+
|
|
|
+o Use rcu_read_lock() and rcu_read_unlock() to guard RCU
|
|
|
+ read-side critical sections.
|
|
|
+
|
|
|
+o Within an RCU read-side critical section, use rcu_dereference()
|
|
|
+ to dereference RCU-protected pointers.
|
|
|
+
|
|
|
+o Use some solid scheme (such as locks or semaphores) to
|
|
|
+ keep concurrent updates from interfering with each other.
|
|
|
+
|
|
|
+o Use rcu_assign_pointer() to update an RCU-protected pointer.
|
|
|
+ This primitive protects concurrent readers from the updater,
|
|
|
+ -not- concurrent updates from each other! You therefore still
|
|
|
+ need to use locking (or something similar) to keep concurrent
|
|
|
+ rcu_assign_pointer() primitives from interfering with each other.
|
|
|
+
|
|
|
+o Use synchronize_rcu() -after- removing a data element from an
|
|
|
+ RCU-protected data structure, but -before- reclaiming/freeing
|
|
|
+ the data element, in order to wait for the completion of all
|
|
|
+ RCU read-side critical sections that might be referencing that
|
|
|
+ data item.
|
|
|
+
|
|
|
+See checklist.txt for additional rules to follow when using RCU.
|
|
|
+
|
|
|
+
|
|
|
+4. WHAT IF MY UPDATING THREAD CANNOT BLOCK?
|
|
|
+
|
|
|
+In the example above, foo_update_a() blocks until a grace period elapses.
|
|
|
+This is quite simple, but in some cases one cannot afford to wait so
|
|
|
+long -- there might be other high-priority work to be done.
|
|
|
+
|
|
|
+In such cases, one uses call_rcu() rather than synchronize_rcu().
|
|
|
+The call_rcu() API is as follows:
|
|
|
+
|
|
|
+ void call_rcu(struct rcu_head * head,
|
|
|
+ void (*func)(struct rcu_head *head));
|
|
|
+
|
|
|
+This function invokes func(head) after a grace period has elapsed.
|
|
|
+This invocation might happen from either softirq or process context,
|
|
|
+so the function is not permitted to block. The foo struct needs to
|
|
|
+have an rcu_head structure added, perhaps as follows:
|
|
|
+
|
|
|
+ struct foo {
|
|
|
+ int a;
|
|
|
+ char b;
|
|
|
+ long c;
|
|
|
+ struct rcu_head rcu;
|
|
|
+ };
|
|
|
+
|
|
|
+The foo_update_a() function might then be written as follows:
|
|
|
+
|
|
|
+ /*
|
|
|
+ * Create a new struct foo that is the same as the one currently
|
|
|
+ * pointed to by gbl_foo, except that field "a" is replaced
|
|
|
+ * with "new_a". Points gbl_foo to the new structure, and
|
|
|
+ * frees up the old structure after a grace period.
|
|
|
+ *
|
|
|
+ * Uses rcu_assign_pointer() to ensure that concurrent readers
|
|
|
+ * see the initialized version of the new structure.
|
|
|
+ *
|
|
|
+ * Uses call_rcu() to ensure that any readers that might have
|
|
|
+ * references to the old structure complete before freeing the
|
|
|
+ * old structure.
|
|
|
+ */
|
|
|
+ void foo_update_a(int new_a)
|
|
|
+ {
|
|
|
+ struct foo *new_fp;
|
|
|
+ struct foo *old_fp;
|
|
|
+
|
|
|
+ new_fp = kmalloc(sizeof(*fp), GFP_KERNEL);
|
|
|
+ spin_lock(&foo_mutex);
|
|
|
+ old_fp = gbl_foo;
|
|
|
+ *new_fp = *old_fp;
|
|
|
+ new_fp->a = new_a;
|
|
|
+ rcu_assign_pointer(gbl_foo, new_fp);
|
|
|
+ spin_unlock(&foo_mutex);
|
|
|
+ call_rcu(&old_fp->rcu, foo_reclaim);
|
|
|
+ }
|
|
|
+
|
|
|
+The foo_reclaim() function might appear as follows:
|
|
|
+
|
|
|
+ void foo_reclaim(struct rcu_head *rp)
|
|
|
+ {
|
|
|
+ struct foo *fp = container_of(rp, struct foo, rcu);
|
|
|
+
|
|
|
+ kfree(fp);
|
|
|
+ }
|
|
|
+
|
|
|
+The container_of() primitive is a macro that, given a pointer into a
|
|
|
+struct, the type of the struct, and the pointed-to field within the
|
|
|
+struct, returns a pointer to the beginning of the struct.
|
|
|
+
|
|
|
+The use of call_rcu() permits the caller of foo_update_a() to
|
|
|
+immediately regain control, without needing to worry further about the
|
|
|
+old version of the newly updated element. It also clearly shows the
|
|
|
+RCU distinction between updater, namely foo_update_a(), and reclaimer,
|
|
|
+namely foo_reclaim().
|
|
|
+
|
|
|
+The summary of advice is the same as for the previous section, except
|
|
|
+that we are now using call_rcu() rather than synchronize_rcu():
|
|
|
+
|
|
|
+o Use call_rcu() -after- removing a data element from an
|
|
|
+ RCU-protected data structure in order to register a callback
|
|
|
+ function that will be invoked after the completion of all RCU
|
|
|
+ read-side critical sections that might be referencing that
|
|
|
+ data item.
|
|
|
+
|
|
|
+Again, see checklist.txt for additional rules governing the use of RCU.
|
|
|
+
|
|
|
+
|
|
|
+5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
|
|
|
+
|
|
|
+One of the nice things about RCU is that it has extremely simple "toy"
|
|
|
+implementations that are a good first step towards understanding the
|
|
|
+production-quality implementations in the Linux kernel. This section
|
|
|
+presents two such "toy" implementations of RCU, one that is implemented
|
|
|
+in terms of familiar locking primitives, and another that more closely
|
|
|
+resembles "classic" RCU. Both are way too simple for real-world use,
|
|
|
+lacking both functionality and performance. However, they are useful
|
|
|
+in getting a feel for how RCU works. See kernel/rcupdate.c for a
|
|
|
+production-quality implementation, and see:
|
|
|
+
|
|
|
+ http://www.rdrop.com/users/paulmck/RCU
|
|
|
+
|
|
|
+for papers describing the Linux kernel RCU implementation. The OLS'01
|
|
|
+and OLS'02 papers are a good introduction, and the dissertation provides
|
|
|
+more details on the current implementation.
|
|
|
+
|
|
|
+
|
|
|
+5A. "TOY" IMPLEMENTATION #1: LOCKING
|
|
|
+
|
|
|
+This section presents a "toy" RCU implementation that is based on
|
|
|
+familiar locking primitives. Its overhead makes it a non-starter for
|
|
|
+real-life use, as does its lack of scalability. It is also unsuitable
|
|
|
+for realtime use, since it allows scheduling latency to "bleed" from
|
|
|
+one read-side critical section to another.
|
|
|
+
|
|
|
+However, it is probably the easiest implementation to relate to, so is
|
|
|
+a good starting point.
|
|
|
+
|
|
|
+It is extremely simple:
|
|
|
+
|
|
|
+ static DEFINE_RWLOCK(rcu_gp_mutex);
|
|
|
+
|
|
|
+ void rcu_read_lock(void)
|
|
|
+ {
|
|
|
+ read_lock(&rcu_gp_mutex);
|
|
|
+ }
|
|
|
+
|
|
|
+ void rcu_read_unlock(void)
|
|
|
+ {
|
|
|
+ read_unlock(&rcu_gp_mutex);
|
|
|
+ }
|
|
|
+
|
|
|
+ void synchronize_rcu(void)
|
|
|
+ {
|
|
|
+ write_lock(&rcu_gp_mutex);
|
|
|
+ write_unlock(&rcu_gp_mutex);
|
|
|
+ }
|
|
|
+
|
|
|
+[You can ignore rcu_assign_pointer() and rcu_dereference() without
|
|
|
+missing much. But here they are anyway. And whatever you do, don't
|
|
|
+forget about them when submitting patches making use of RCU!]
|
|
|
+
|
|
|
+ #define rcu_assign_pointer(p, v) ({ \
|
|
|
+ smp_wmb(); \
|
|
|
+ (p) = (v); \
|
|
|
+ })
|
|
|
+
|
|
|
+ #define rcu_dereference(p) ({ \
|
|
|
+ typeof(p) _________p1 = p; \
|
|
|
+ smp_read_barrier_depends(); \
|
|
|
+ (_________p1); \
|
|
|
+ })
|
|
|
+
|
|
|
+
|
|
|
+The rcu_read_lock() and rcu_read_unlock() primitive read-acquire
|
|
|
+and release a global reader-writer lock. The synchronize_rcu()
|
|
|
+primitive write-acquires this same lock, then immediately releases
|
|
|
+it. This means that once synchronize_rcu() exits, all RCU read-side
|
|
|
+critical sections that were in progress before synchonize_rcu() was
|
|
|
+called are guaranteed to have completed -- there is no way that
|
|
|
+synchronize_rcu() would have been able to write-acquire the lock
|
|
|
+otherwise.
|
|
|
+
|
|
|
+It is possible to nest rcu_read_lock(), since reader-writer locks may
|
|
|
+be recursively acquired. Note also that rcu_read_lock() is immune
|
|
|
+from deadlock (an important property of RCU). The reason for this is
|
|
|
+that the only thing that can block rcu_read_lock() is a synchronize_rcu().
|
|
|
+But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex,
|
|
|
+so there can be no deadlock cycle.
|
|
|
+
|
|
|
+Quick Quiz #1: Why is this argument naive? How could a deadlock
|
|
|
+ occur when using this algorithm in a real-world Linux
|
|
|
+ kernel? How could this deadlock be avoided?
|
|
|
+
|
|
|
+
|
|
|
+5B. "TOY" EXAMPLE #2: CLASSIC RCU
|
|
|
+
|
|
|
+This section presents a "toy" RCU implementation that is based on
|
|
|
+"classic RCU". It is also short on performance (but only for updates) and
|
|
|
+on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT
|
|
|
+kernels. The definitions of rcu_dereference() and rcu_assign_pointer()
|
|
|
+are the same as those shown in the preceding section, so they are omitted.
|
|
|
+
|
|
|
+ void rcu_read_lock(void) { }
|
|
|
+
|
|
|
+ void rcu_read_unlock(void) { }
|
|
|
+
|
|
|
+ void synchronize_rcu(void)
|
|
|
+ {
|
|
|
+ int cpu;
|
|
|
+
|
|
|
+ for_each_cpu(cpu)
|
|
|
+ run_on(cpu);
|
|
|
+ }
|
|
|
+
|
|
|
+Note that rcu_read_lock() and rcu_read_unlock() do absolutely nothing.
|
|
|
+This is the great strength of classic RCU in a non-preemptive kernel:
|
|
|
+read-side overhead is precisely zero, at least on non-Alpha CPUs.
|
|
|
+And there is absolutely no way that rcu_read_lock() can possibly
|
|
|
+participate in a deadlock cycle!
|
|
|
+
|
|
|
+The implementation of synchronize_rcu() simply schedules itself on each
|
|
|
+CPU in turn. The run_on() primitive can be implemented straightforwardly
|
|
|
+in terms of the sched_setaffinity() primitive. Of course, a somewhat less
|
|
|
+"toy" implementation would restore the affinity upon completion rather
|
|
|
+than just leaving all tasks running on the last CPU, but when I said
|
|
|
+"toy", I meant -toy-!
|
|
|
+
|
|
|
+So how the heck is this supposed to work???
|
|
|
+
|
|
|
+Remember that it is illegal to block while in an RCU read-side critical
|
|
|
+section. Therefore, if a given CPU executes a context switch, we know
|
|
|
+that it must have completed all preceding RCU read-side critical sections.
|
|
|
+Once -all- CPUs have executed a context switch, then -all- preceding
|
|
|
+RCU read-side critical sections will have completed.
|
|
|
+
|
|
|
+So, suppose that we remove a data item from its structure and then invoke
|
|
|
+synchronize_rcu(). Once synchronize_rcu() returns, we are guaranteed
|
|
|
+that there are no RCU read-side critical sections holding a reference
|
|
|
+to that data item, so we can safely reclaim it.
|
|
|
+
|
|
|
+Quick Quiz #2: Give an example where Classic RCU's read-side
|
|
|
+ overhead is -negative-.
|
|
|
+
|
|
|
+Quick Quiz #3: If it is illegal to block in an RCU read-side
|
|
|
+ critical section, what the heck do you do in
|
|
|
+ PREEMPT_RT, where normal spinlocks can block???
|
|
|
+
|
|
|
+
|
|
|
+6. ANALOGY WITH READER-WRITER LOCKING
|
|
|
+
|
|
|
+Although RCU can be used in many different ways, a very common use of
|
|
|
+RCU is analogous to reader-writer locking. The following unified
|
|
|
+diff shows how closely related RCU and reader-writer locking can be.
|
|
|
+
|
|
|
+ @@ -13,15 +14,15 @@
|
|
|
+ struct list_head *lp;
|
|
|
+ struct el *p;
|
|
|
+
|
|
|
+ - read_lock();
|
|
|
+ - list_for_each_entry(p, head, lp) {
|
|
|
+ + rcu_read_lock();
|
|
|
+ + list_for_each_entry_rcu(p, head, lp) {
|
|
|
+ if (p->key == key) {
|
|
|
+ *result = p->data;
|
|
|
+ - read_unlock();
|
|
|
+ + rcu_read_unlock();
|
|
|
+ return 1;
|
|
|
+ }
|
|
|
+ }
|
|
|
+ - read_unlock();
|
|
|
+ + rcu_read_unlock();
|
|
|
+ return 0;
|
|
|
+ }
|
|
|
+
|
|
|
+ @@ -29,15 +30,16 @@
|
|
|
+ {
|
|
|
+ struct el *p;
|
|
|
+
|
|
|
+ - write_lock(&listmutex);
|
|
|
+ + spin_lock(&listmutex);
|
|
|
+ list_for_each_entry(p, head, lp) {
|
|
|
+ if (p->key == key) {
|
|
|
+ list_del(&p->list);
|
|
|
+ - write_unlock(&listmutex);
|
|
|
+ + spin_unlock(&listmutex);
|
|
|
+ + synchronize_rcu();
|
|
|
+ kfree(p);
|
|
|
+ return 1;
|
|
|
+ }
|
|
|
+ }
|
|
|
+ - write_unlock(&listmutex);
|
|
|
+ + spin_unlock(&listmutex);
|
|
|
+ return 0;
|
|
|
+ }
|
|
|
+
|
|
|
+Or, for those who prefer a side-by-side listing:
|
|
|
+
|
|
|
+ 1 struct el { 1 struct el {
|
|
|
+ 2 struct list_head list; 2 struct list_head list;
|
|
|
+ 3 long key; 3 long key;
|
|
|
+ 4 spinlock_t mutex; 4 spinlock_t mutex;
|
|
|
+ 5 int data; 5 int data;
|
|
|
+ 6 /* Other data fields */ 6 /* Other data fields */
|
|
|
+ 7 }; 7 };
|
|
|
+ 8 spinlock_t listmutex; 8 spinlock_t listmutex;
|
|
|
+ 9 struct el head; 9 struct el head;
|
|
|
+
|
|
|
+ 1 int search(long key, int *result) 1 int search(long key, int *result)
|
|
|
+ 2 { 2 {
|
|
|
+ 3 struct list_head *lp; 3 struct list_head *lp;
|
|
|
+ 4 struct el *p; 4 struct el *p;
|
|
|
+ 5 5
|
|
|
+ 6 read_lock(); 6 rcu_read_lock();
|
|
|
+ 7 list_for_each_entry(p, head, lp) { 7 list_for_each_entry_rcu(p, head, lp) {
|
|
|
+ 8 if (p->key == key) { 8 if (p->key == key) {
|
|
|
+ 9 *result = p->data; 9 *result = p->data;
|
|
|
+10 read_unlock(); 10 rcu_read_unlock();
|
|
|
+11 return 1; 11 return 1;
|
|
|
+12 } 12 }
|
|
|
+13 } 13 }
|
|
|
+14 read_unlock(); 14 rcu_read_unlock();
|
|
|
+15 return 0; 15 return 0;
|
|
|
+16 } 16 }
|
|
|
+
|
|
|
+ 1 int delete(long key) 1 int delete(long key)
|
|
|
+ 2 { 2 {
|
|
|
+ 3 struct el *p; 3 struct el *p;
|
|
|
+ 4 4
|
|
|
+ 5 write_lock(&listmutex); 5 spin_lock(&listmutex);
|
|
|
+ 6 list_for_each_entry(p, head, lp) { 6 list_for_each_entry(p, head, lp) {
|
|
|
+ 7 if (p->key == key) { 7 if (p->key == key) {
|
|
|
+ 8 list_del(&p->list); 8 list_del(&p->list);
|
|
|
+ 9 write_unlock(&listmutex); 9 spin_unlock(&listmutex);
|
|
|
+ 10 synchronize_rcu();
|
|
|
+10 kfree(p); 11 kfree(p);
|
|
|
+11 return 1; 12 return 1;
|
|
|
+12 } 13 }
|
|
|
+13 } 14 }
|
|
|
+14 write_unlock(&listmutex); 15 spin_unlock(&listmutex);
|
|
|
+15 return 0; 16 return 0;
|
|
|
+16 } 17 }
|
|
|
+
|
|
|
+Either way, the differences are quite small. Read-side locking moves
|
|
|
+to rcu_read_lock() and rcu_read_unlock, update-side locking moves from
|
|
|
+from a reader-writer lock to a simple spinlock, and a synchronize_rcu()
|
|
|
+precedes the kfree().
|
|
|
+
|
|
|
+However, there is one potential catch: the read-side and update-side
|
|
|
+critical sections can now run concurrently. In many cases, this will
|
|
|
+not be a problem, but it is necessary to check carefully regardless.
|
|
|
+For example, if multiple independent list updates must be seen as
|
|
|
+a single atomic update, converting to RCU will require special care.
|
|
|
+
|
|
|
+Also, the presence of synchronize_rcu() means that the RCU version of
|
|
|
+delete() can now block. If this is a problem, there is a callback-based
|
|
|
+mechanism that never blocks, namely call_rcu(), that can be used in
|
|
|
+place of synchronize_rcu().
|
|
|
+
|
|
|
+
|
|
|
+7. FULL LIST OF RCU APIs
|
|
|
+
|
|
|
+The RCU APIs are documented in docbook-format header comments in the
|
|
|
+Linux-kernel source code, but it helps to have a full list of the
|
|
|
+APIs, since there does not appear to be a way to categorize them
|
|
|
+in docbook. Here is the list, by category.
|
|
|
+
|
|
|
+Markers for RCU read-side critical sections:
|
|
|
+
|
|
|
+ rcu_read_lock
|
|
|
+ rcu_read_unlock
|
|
|
+ rcu_read_lock_bh
|
|
|
+ rcu_read_unlock_bh
|
|
|
+
|
|
|
+RCU pointer/list traversal:
|
|
|
+
|
|
|
+ rcu_dereference
|
|
|
+ list_for_each_rcu (to be deprecated in favor of
|
|
|
+ list_for_each_entry_rcu)
|
|
|
+ list_for_each_safe_rcu (deprecated, not used)
|
|
|
+ list_for_each_entry_rcu
|
|
|
+ list_for_each_continue_rcu (to be deprecated in favor of new
|
|
|
+ list_for_each_entry_continue_rcu)
|
|
|
+ hlist_for_each_rcu (to be deprecated in favor of
|
|
|
+ hlist_for_each_entry_rcu)
|
|
|
+ hlist_for_each_entry_rcu
|
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+
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+RCU pointer update:
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+
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+ rcu_assign_pointer
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+ list_add_rcu
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+ list_add_tail_rcu
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+ list_del_rcu
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+ list_replace_rcu
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+ hlist_del_rcu
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+ hlist_add_head_rcu
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+
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+RCU grace period:
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+
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+ synchronize_kernel (deprecated)
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+ synchronize_net
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+ synchronize_sched
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+ synchronize_rcu
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+ call_rcu
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+ call_rcu_bh
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+
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+See the comment headers in the source code (or the docbook generated
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+from them) for more information.
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+
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+
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+8. ANSWERS TO QUICK QUIZZES
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+
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+Quick Quiz #1: Why is this argument naive? How could a deadlock
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+ occur when using this algorithm in a real-world Linux
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+ kernel? [Referring to the lock-based "toy" RCU
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+ algorithm.]
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+
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+Answer: Consider the following sequence of events:
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+
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+ 1. CPU 0 acquires some unrelated lock, call it
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+ "problematic_lock".
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+
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+ 2. CPU 1 enters synchronize_rcu(), write-acquiring
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+ rcu_gp_mutex.
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+
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+ 3. CPU 0 enters rcu_read_lock(), but must wait
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+ because CPU 1 holds rcu_gp_mutex.
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+
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+ 4. CPU 1 is interrupted, and the irq handler
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+ attempts to acquire problematic_lock.
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+
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+ The system is now deadlocked.
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+
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+ One way to avoid this deadlock is to use an approach like
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+ that of CONFIG_PREEMPT_RT, where all normal spinlocks
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+ become blocking locks, and all irq handlers execute in
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+ the context of special tasks. In this case, in step 4
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+ above, the irq handler would block, allowing CPU 1 to
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+ release rcu_gp_mutex, avoiding the deadlock.
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+
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+ Even in the absence of deadlock, this RCU implementation
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+ allows latency to "bleed" from readers to other
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+ readers through synchronize_rcu(). To see this,
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+ consider task A in an RCU read-side critical section
|
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|
+ (thus read-holding rcu_gp_mutex), task B blocked
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|
+ attempting to write-acquire rcu_gp_mutex, and
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+ task C blocked in rcu_read_lock() attempting to
|
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+ read_acquire rcu_gp_mutex. Task A's RCU read-side
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|
+ latency is holding up task C, albeit indirectly via
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|
+ task B.
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+
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+ Realtime RCU implementations therefore use a counter-based
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|
+ approach where tasks in RCU read-side critical sections
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|
+ cannot be blocked by tasks executing synchronize_rcu().
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+
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+Quick Quiz #2: Give an example where Classic RCU's read-side
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|
+ overhead is -negative-.
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+
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|
+Answer: Imagine a single-CPU system with a non-CONFIG_PREEMPT
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|
+ kernel where a routing table is used by process-context
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|
+ code, but can be updated by irq-context code (for example,
|
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|
+ by an "ICMP REDIRECT" packet). The usual way of handling
|
|
|
+ this would be to have the process-context code disable
|
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|
+ interrupts while searching the routing table. Use of
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|
+ RCU allows such interrupt-disabling to be dispensed with.
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|
+ Thus, without RCU, you pay the cost of disabling interrupts,
|
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|
+ and with RCU you don't.
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+
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|
+ One can argue that the overhead of RCU in this
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|
|
+ case is negative with respect to the single-CPU
|
|
|
+ interrupt-disabling approach. Others might argue that
|
|
|
+ the overhead of RCU is merely zero, and that replacing
|
|
|
+ the positive overhead of the interrupt-disabling scheme
|
|
|
+ with the zero-overhead RCU scheme does not constitute
|
|
|
+ negative overhead.
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|
+
|
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|
+ In real life, of course, things are more complex. But
|
|
|
+ even the theoretical possibility of negative overhead for
|
|
|
+ a synchronization primitive is a bit unexpected. ;-)
|
|
|
+
|
|
|
+Quick Quiz #3: If it is illegal to block in an RCU read-side
|
|
|
+ critical section, what the heck do you do in
|
|
|
+ PREEMPT_RT, where normal spinlocks can block???
|
|
|
+
|
|
|
+Answer: Just as PREEMPT_RT permits preemption of spinlock
|
|
|
+ critical sections, it permits preemption of RCU
|
|
|
+ read-side critical sections. It also permits
|
|
|
+ spinlocks blocking while in RCU read-side critical
|
|
|
+ sections.
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|
+
|
|
|
+ Why the apparent inconsistency? Because it is it
|
|
|
+ possible to use priority boosting to keep the RCU
|
|
|
+ grace periods short if need be (for example, if running
|
|
|
+ short of memory). In contrast, if blocking waiting
|
|
|
+ for (say) network reception, there is no way to know
|
|
|
+ what should be boosted. Especially given that the
|
|
|
+ process we need to boost might well be a human being
|
|
|
+ who just went out for a pizza or something. And although
|
|
|
+ a computer-operated cattle prod might arouse serious
|
|
|
+ interest, it might also provoke serious objections.
|
|
|
+ Besides, how does the computer know what pizza parlor
|
|
|
+ the human being went to???
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|
+
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+
|
|
|
+ACKNOWLEDGEMENTS
|
|
|
+
|
|
|
+My thanks to the people who helped make this human-readable, including
|
|
|
+Jon Walpole, Josh Triplett, Serge Hallyn, and Suzanne Wood.
|
|
|
+
|
|
|
+
|
|
|
+For more information, see http://www.rdrop.com/users/paulmck/RCU.
|
Paul E. McKenney