poll: add poll_requested_events() and poll_does_not_wait() functions

In some cases the poll() implementation in a driver has to do different
things depending on the events the caller wants to poll for.  An example
is when a driver needs to start a DMA engine if the caller polls for
POLLIN, but doesn't want to do that if POLLIN is not requested but instead
only POLLOUT or POLLPRI is requested.  This is something that can happen
in the video4linux subsystem among others.

Unfortunately, the current epoll/poll/select implementation doesn't
provide that information reliably.  The poll_table_struct does have it: it
has a key field with the event mask.  But once a poll() call matches one
or more bits of that mask any following poll() calls are passed a NULL
poll_table pointer.

Also, the eventpoll implementation always left the key field at ~0 instead
of using the requested events mask.

This was changed in eventpoll.c so the key field now contains the actual
events that should be polled for as set by the caller.

The solution to the NULL poll_table pointer is to set the qproc field to
NULL in poll_table once poll() matches the events, not the poll_table
pointer itself.  That way drivers can obtain the mask through a new
poll_requested_events inline.

The poll_table_struct can still be NULL since some kernel code calls it
internally (netfs_state_poll() in ./drivers/staging/pohmelfs/netfs.h).  In
that case poll_requested_events() returns ~0 (i.e.  all events).

Very rarely drivers might want to know whether poll_wait will actually
wait.  If another earlier file descriptor in the set already matched the
events the caller wanted to wait for, then the kernel will return from the
select() call without waiting.  This might be useful information in order
to avoid doing expensive work.

A new helper function poll_does_not_wait() is added that drivers can use
to detect this situation.  This is now used in sock_poll_wait() in
include/net/sock.h.  This was the only place in the kernel that needed
this information.

Drivers should no longer access any of the poll_table internals, but use
the poll_requested_events() and poll_does_not_wait() access functions
instead.  In order to enforce that the poll_table fields are now prepended
with an underscore and a comment was added warning against using them
directly.

This required a change in unix_dgram_poll() in unix/af_unix.c which used
the key field to get the requested events.  It's been replaced by a call
to poll_requested_events().

For qproc it was especially important to change its name since the
behavior of that field changes with this patch since this function pointer
can now be NULL when that wasn't possible in the past.

Any driver accessing the qproc or key fields directly will now fail to compile.

Some notes regarding the correctness of this patch: the driver's poll()
function is called with a 'struct poll_table_struct *wait' argument.  This
pointer may or may not be NULL, drivers can never rely on it being one or
the other as that depends on whether or not an earlier file descriptor in
the select()'s fdset matched the requested events.

There are only three things a driver can do with the wait argument:

1) obtain the key field:

	events = wait ? wait->key : ~0;

   This will still work although it should be replaced with the new
   poll_requested_events() function (which does exactly the same).
   This will now even work better, since wait is no longer set to NULL
   unnecessarily.

2) use the qproc callback. This could be deadly since qproc can now be
   NULL. Renaming qproc should prevent this from happening. There are no
   kernel drivers that actually access this callback directly, BTW.

3) test whether wait == NULL to determine whether poll would return without
   waiting. This is no longer sufficient as the correct test is now
   wait == NULL || wait->_qproc == NULL.

   However, the worst that can happen here is a slight performance hit in
   the case where wait != NULL and wait->_qproc == NULL. In that case the
   driver will assume that poll_wait() will actually add the fd to the set
   of waiting file descriptors. Of course, poll_wait() will not do that
   since it tests for wait->_qproc. This will not break anything, though.

   There is only one place in the whole kernel where this happens
   (sock_poll_wait() in include/net/sock.h) and that code will be replaced
   by a call to poll_does_not_wait() in the next patch.

   Note that even if wait->_qproc != NULL drivers cannot rely on poll_wait()
   actually waiting. The next file descriptor from the set might match the
   event mask and thus any possible waits will never happen.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Jonathan Corbet <corbet@lwn.net>
Reviewed-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Davide Libenzi <davidel@xmailserver.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Cc: Mauro Carvalho Chehab <mchehab@infradead.org>
Cc: David Miller <davem@davemloft.net>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/eventpoll.c

@@ -699,9 +699,12 @@ static int ep_read_events_proc(struct eventpoll *ep, struct list_head *head,
 			       void *priv)
 {
 	struct epitem *epi, *tmp;
+	poll_table pt;
 
+	init_poll_funcptr(&pt, NULL);
 	list_for_each_entry_safe(epi, tmp, head, rdllink) {
-		if (epi->ffd.file->f_op->poll(epi->ffd.file, NULL) &
+		pt._key = epi->event.events;
+		if (epi->ffd.file->f_op->poll(epi->ffd.file, &pt) &
 		    epi->event.events)
 			return POLLIN | POLLRDNORM;
 		else {
@@ -1097,6 +1100,7 @@ static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
 	/* Initialize the poll table using the queue callback */
 	epq.epi = epi;
 	init_poll_funcptr(&epq.pt, ep_ptable_queue_proc);
+	epq.pt._key = event->events;
 
 	/*
 	 * Attach the item to the poll hooks and get current event bits.
@@ -1191,6 +1195,9 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_even
 {
 	int pwake = 0;
 	unsigned int revents;
+	poll_table pt;
+
+	init_poll_funcptr(&pt, NULL);
 
 	/*
 	 * Set the new event interest mask before calling f_op->poll();
@@ -1198,13 +1205,14 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_even
 	 * f_op->poll() call and the new event set registering.
 	 */
 	epi->event.events = event->events;
+	pt._key = event->events;
 	epi->event.data = event->data; /* protected by mtx */
 
 	/*
 	 * Get current event bits. We can safely use the file* here because
 	 * its usage count has been increased by the caller of this function.
 	 */
-	revents = epi->ffd.file->f_op->poll(epi->ffd.file, NULL);
+	revents = epi->ffd.file->f_op->poll(epi->ffd.file, &pt);
 
 	/*
 	 * If the item is "hot" and it is not registered inside the ready
@@ -1239,6 +1247,9 @@ static int ep_send_events_proc(struct eventpoll *ep, struct list_head *head,
 	unsigned int revents;
 	struct epitem *epi;
 	struct epoll_event __user *uevent;
+	poll_table pt;
+
+	init_poll_funcptr(&pt, NULL);
 
 	/*
 	 * We can loop without lock because we are passed a task private list.
@@ -1251,7 +1262,8 @@ static int ep_send_events_proc(struct eventpoll *ep, struct list_head *head,
 
 		list_del_init(&epi->rdllink);
 
-		revents = epi->ffd.file->f_op->poll(epi->ffd.file, NULL) &
+		pt._key = epi->event.events;
+		revents = epi->ffd.file->f_op->poll(epi->ffd.file, &pt) &
 			epi->event.events;
 
 		/*

fs/select.c

@@ -223,7 +223,7 @@ static void __pollwait(struct file *filp, wait_queue_head_t *wait_address,
 	get_file(filp);
 	entry->filp = filp;
 	entry->wait_address = wait_address;
-	entry->key = p->key;
+	entry->key = p->_key;
 	init_waitqueue_func_entry(&entry->wait, pollwake);
 	entry->wait.private = pwq;
 	add_wait_queue(wait_address, &entry->wait);
@@ -386,13 +386,11 @@ get_max:
 static inline void wait_key_set(poll_table *wait, unsigned long in,
 				unsigned long out, unsigned long bit)
 {
-	if (wait) {
-		wait->key = POLLEX_SET;
-		if (in & bit)
-			wait->key |= POLLIN_SET;
-		if (out & bit)
-			wait->key |= POLLOUT_SET;
-	}
+	wait->_key = POLLEX_SET;
+	if (in & bit)
+		wait->_key |= POLLIN_SET;
+	if (out & bit)
+		wait->_key |= POLLOUT_SET;
 }
 
 int do_select(int n, fd_set_bits *fds, struct timespec *end_time)
@@ -414,7 +412,7 @@ int do_select(int n, fd_set_bits *fds, struct timespec *end_time)
 	poll_initwait(&table);
 	wait = &table.pt;
 	if (end_time && !end_time->tv_sec && !end_time->tv_nsec) {
-		wait = NULL;
+		wait->_qproc = NULL;
 		timed_out = 1;
 	}
 
@@ -459,17 +457,17 @@ int do_select(int n, fd_set_bits *fds, struct timespec *end_time)
 					if ((mask & POLLIN_SET) && (in & bit)) {
 						res_in |= bit;
 						retval++;
-						wait = NULL;
+						wait->_qproc = NULL;
 					}
 					if ((mask & POLLOUT_SET) && (out & bit)) {
 						res_out |= bit;
 						retval++;
-						wait = NULL;
+						wait->_qproc = NULL;
 					}
 					if ((mask & POLLEX_SET) && (ex & bit)) {
 						res_ex |= bit;
 						retval++;
-						wait = NULL;
+						wait->_qproc = NULL;
 					}
 				}
 			}
@@ -481,7 +479,7 @@ int do_select(int n, fd_set_bits *fds, struct timespec *end_time)
 				*rexp = res_ex;
 			cond_resched();
 		}
-		wait = NULL;
+		wait->_qproc = NULL;
 		if (retval || timed_out || signal_pending(current))
 			break;
 		if (table.error) {
@@ -720,7 +718,7 @@ struct poll_list {
  * interested in events matching the pollfd->events mask, and the result
  * matching that mask is both recorded in pollfd->revents and returned. The
  * pwait poll_table will be used by the fd-provided poll handler for waiting,
- * if non-NULL.
+ * if pwait->_qproc is non-NULL.
  */
 static inline unsigned int do_pollfd(struct pollfd *pollfd, poll_table *pwait)
 {
@@ -738,9 +736,7 @@ static inline unsigned int do_pollfd(struct pollfd *pollfd, poll_table *pwait)
 		if (file != NULL) {
 			mask = DEFAULT_POLLMASK;
 			if (file->f_op && file->f_op->poll) {
-				if (pwait)
-					pwait->key = pollfd->events |
-							POLLERR | POLLHUP;
+				pwait->_key = pollfd->events|POLLERR|POLLHUP;
 				mask = file->f_op->poll(file, pwait);
 			}
 			/* Mask out unneeded events. */
@@ -763,7 +759,7 @@ static int do_poll(unsigned int nfds,  struct poll_list *list,
 
 	/* Optimise the no-wait case */
 	if (end_time && !end_time->tv_sec && !end_time->tv_nsec) {
-		pt = NULL;
+		pt->_qproc = NULL;
 		timed_out = 1;
 	}
 
@@ -781,22 +777,22 @@ static int do_poll(unsigned int nfds,  struct poll_list *list,
 			for (; pfd != pfd_end; pfd++) {
 				/*
 				 * Fish for events. If we found one, record it
-				 * and kill the poll_table, so we don't
+				 * and kill poll_table->_qproc, so we don't
 				 * needlessly register any other waiters after
 				 * this. They'll get immediately deregistered
 				 * when we break out and return.
 				 */
 				if (do_pollfd(pfd, pt)) {
 					count++;
-					pt = NULL;
+					pt->_qproc = NULL;
 				}
 			}
 		}
 		/*
 		 * All waiters have already been registered, so don't provide
-		 * a poll_table to them on the next loop iteration.
+		 * a poll_table->_qproc to them on the next loop iteration.
 		 */
-		pt = NULL;
+		pt->_qproc = NULL;
 		if (!count) {
 			count = wait->error;
 			if (signal_pending(current))

include/linux/poll.h

@@ -32,21 +32,46 @@ struct poll_table_struct;
  */
 typedef void (*poll_queue_proc)(struct file *, wait_queue_head_t *, struct poll_table_struct *);
 
+/*
+ * Do not touch the structure directly, use the access functions
+ * poll_does_not_wait() and poll_requested_events() instead.
+ */
 typedef struct poll_table_struct {
-	poll_queue_proc qproc;
-	unsigned long key;
+	poll_queue_proc _qproc;
+	unsigned long _key;
 } poll_table;
 
 static inline void poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p)
 {
-	if (p && wait_address)
-		p->qproc(filp, wait_address, p);
+	if (p && p->_qproc && wait_address)
+		p->_qproc(filp, wait_address, p);
+}
+
+/*
+ * Return true if it is guaranteed that poll will not wait. This is the case
+ * if the poll() of another file descriptor in the set got an event, so there
+ * is no need for waiting.
+ */
+static inline bool poll_does_not_wait(const poll_table *p)
+{
+	return p == NULL || p->_qproc == NULL;
+}
+
+/*
+ * Return the set of events that the application wants to poll for.
+ * This is useful for drivers that need to know whether a DMA transfer has
+ * to be started implicitly on poll(). You typically only want to do that
+ * if the application is actually polling for POLLIN and/or POLLOUT.
+ */
+static inline unsigned long poll_requested_events(const poll_table *p)
+{
+	return p ? p->_key : ~0UL;
 }
 
 static inline void init_poll_funcptr(poll_table *pt, poll_queue_proc qproc)
 {
-	pt->qproc = qproc;
-	pt->key   = ~0UL; /* all events enabled */
+	pt->_qproc = qproc;
+	pt->_key   = ~0UL; /* all events enabled */
 }
 
 struct poll_table_entry {

include/net/sock.h

@@ -1854,7 +1854,7 @@ static inline bool wq_has_sleeper(struct socket_wq *wq)
 static inline void sock_poll_wait(struct file *filp,
 		wait_queue_head_t *wait_address, poll_table *p)
 {
-	if (p && wait_address) {
+	if (!poll_does_not_wait(p) && wait_address) {
 		poll_wait(filp, wait_address, p);
 		/*
 		 * We need to be sure we are in sync with the

net/unix/af_unix.c

@@ -2206,7 +2206,7 @@ static unsigned int unix_dgram_poll(struct file *file, struct socket *sock,
 	}
 
 	/* No write status requested, avoid expensive OUT tests. */
-	if (wait && !(wait->key & (POLLWRBAND | POLLWRNORM | POLLOUT)))
+	if (!(poll_requested_events(wait) & (POLLWRBAND|POLLWRNORM|POLLOUT)))
 		return mask;
 
 	writable = unix_writable(sk);