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- RCU and lockdep checking
- All flavors of RCU have lockdep checking available, so that lockdep is
- aware of when each task enters and leaves any flavor of RCU read-side
- critical section. Each flavor of RCU is tracked separately (but note
- that this is not the case in 2.6.32 and earlier). This allows lockdep's
- tracking to include RCU state, which can sometimes help when debugging
- deadlocks and the like.
- In addition, RCU provides the following primitives that check lockdep's
- state:
- rcu_read_lock_held() for normal RCU.
- rcu_read_lock_bh_held() for RCU-bh.
- rcu_read_lock_sched_held() for RCU-sched.
- srcu_read_lock_held() for SRCU.
- These functions are conservative, and will therefore return 1 if they
- aren't certain (for example, if CONFIG_DEBUG_LOCK_ALLOC is not set).
- This prevents things like WARN_ON(!rcu_read_lock_held()) from giving false
- positives when lockdep is disabled.
- In addition, a separate kernel config parameter CONFIG_PROVE_RCU enables
- checking of rcu_dereference() primitives:
- rcu_dereference(p):
- Check for RCU read-side critical section.
- rcu_dereference_bh(p):
- Check for RCU-bh read-side critical section.
- rcu_dereference_sched(p):
- Check for RCU-sched read-side critical section.
- srcu_dereference(p, sp):
- Check for SRCU read-side critical section.
- rcu_dereference_check(p, c):
- Use explicit check expression "c".
- rcu_dereference_raw(p)
- Don't check. (Use sparingly, if at all.)
- The rcu_dereference_check() check expression can be any boolean
- expression, but would normally include one of the rcu_read_lock_held()
- family of functions and a lockdep expression. However, any boolean
- expression can be used. For a moderately ornate example, consider
- the following:
- file = rcu_dereference_check(fdt->fd[fd],
- rcu_read_lock_held() ||
- lockdep_is_held(&files->file_lock) ||
- atomic_read(&files->count) == 1);
- This expression picks up the pointer "fdt->fd[fd]" in an RCU-safe manner,
- and, if CONFIG_PROVE_RCU is configured, verifies that this expression
- is used in:
- 1. An RCU read-side critical section, or
- 2. with files->file_lock held, or
- 3. on an unshared files_struct.
- In case (1), the pointer is picked up in an RCU-safe manner for vanilla
- RCU read-side critical sections, in case (2) the ->file_lock prevents
- any change from taking place, and finally, in case (3) the current task
- is the only task accessing the file_struct, again preventing any change
- from taking place.
- There are currently only "universal" versions of the rcu_assign_pointer()
- and RCU list-/tree-traversal primitives, which do not (yet) check for
- being in an RCU read-side critical section. In the future, separate
- versions of these primitives might be created.
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