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Commit 57a173bd authored by Jason Baron's avatar Jason Baron Committed by Linus Torvalds
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epoll: avoid calling ep_call_nested() from ep_poll_safewake() 

ep_poll_safewake() is used to wakeup potentially nested epoll file
descriptors.  The function uses ep_call_nested() to prevent entering the
same wake up queue more than once, and to prevent excessively deep
wakeup paths (deeper than EP_MAX_NESTS).  However, this is not necessary
since we are already preventing these conditions during EPOLL_CTL_ADD.
This saves extra function calls, and avoids taking a global lock during
the ep_call_nested() calls.

I have, however, left ep_call_nested() for the CONFIG_DEBUG_LOCK_ALLOC
case, since ep_call_nested() keeps track of the nesting level, and this
is required by the call to spin_lock_irqsave_nested().  It would be nice
to remove the ep_call_nested() calls for the CONFIG_DEBUG_LOCK_ALLOC
case as well, however its not clear how to simply pass the nesting level
through multiple wake_up() levels without more surgery.  In any case, I
don't think CONFIG_DEBUG_LOCK_ALLOC is generally used for production.
This patch, also apparently fixes a workload at Google that Salman Qazi
reported by completely removing the poll_safewake_ncalls->lock from
wakeup paths.

Link: http://lkml.kernel.org/r/1507920533-8812-1-git-send-email-jbaron@akamai.com


Signed-off-by: default avatarJason Baron <jbaron@akamai.com>
Acked-by: default avatarDavidlohr Bueso <dbueso@suse.de>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Salman Qazi <sqazi@google.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 2ae928a9

fs/eventpoll.c

+18 −29
Original line number Diff line number Diff line
@@ -276,9 +276,6 @@ static DEFINE_MUTEX(epmutex); 
/* Used to check for epoll file descriptor inclusion loops */

static struct nested_calls poll_loop_ncalls;



/* Used for safe wake up implementation */

static struct nested_calls poll_safewake_ncalls;



/* Used to call file's f_op->poll() under the nested calls boundaries */

static struct nested_calls poll_readywalk_ncalls;
@@ -551,40 +548,21 @@ static int ep_call_nested(struct nested_calls *ncalls, int max_nests, 
 * this special case of epoll.

 */

#ifdef CONFIG_DEBUG_LOCK_ALLOC

static inline void ep_wake_up_nested(wait_queue_head_t *wqueue,

				     unsigned long events, int subclass)



static struct nested_calls poll_safewake_ncalls;



static int ep_poll_wakeup_proc(void *priv, void *cookie, int call_nests)

{

	unsigned long flags;

	wait_queue_head_t *wqueue = (wait_queue_head_t *)cookie;



	spin_lock_irqsave_nested(&wqueue->lock, flags, subclass);

	wake_up_locked_poll(wqueue, events);

	spin_lock_irqsave_nested(&wqueue->lock, flags, call_nests + 1);

	wake_up_locked_poll(wqueue, POLLIN);

	spin_unlock_irqrestore(&wqueue->lock, flags);

}

#else

static inline void ep_wake_up_nested(wait_queue_head_t *wqueue,

				     unsigned long events, int subclass)

{

	wake_up_poll(wqueue, events);

}

#endif



static int ep_poll_wakeup_proc(void *priv, void *cookie, int call_nests)

{

	ep_wake_up_nested((wait_queue_head_t *) cookie, POLLIN,

			  1 + call_nests);

	return 0;

}



/*

 * Perform a safe wake up of the poll wait list. The problem is that

 * with the new callback'd wake up system, it is possible that the

 * poll callback is reentered from inside the call to wake_up() done

 * on the poll wait queue head. The rule is that we cannot reenter the

 * wake up code from the same task more than EP_MAX_NESTS times,

 * and we cannot reenter the same wait queue head at all. This will

 * enable to have a hierarchy of epoll file descriptor of no more than

 * EP_MAX_NESTS deep.

 */

static void ep_poll_safewake(wait_queue_head_t *wq)

{

	int this_cpu = get_cpu();
@@ -595,6 +573,15 @@ static void ep_poll_safewake(wait_queue_head_t *wq) 
	put_cpu();

}



#else



static void ep_poll_safewake(wait_queue_head_t *wq)

{

	wake_up_poll(wq, POLLIN);

}



#endif



static void ep_remove_wait_queue(struct eppoll_entry *pwq)

{

	wait_queue_head_t *whead;
@@ -2315,8 +2302,10 @@ static int __init eventpoll_init(void) 
	 */

	ep_nested_calls_init(&poll_loop_ncalls);



#ifdef CONFIG_DEBUG_LOCK_ALLOC

	/* Initialize the structure used to perform safe poll wait head wake ups */

	ep_nested_calls_init(&poll_safewake_ncalls);

#endif



	/* Initialize the structure used to perform file's f_op->poll() calls */

	ep_nested_calls_init(&poll_readywalk_ncalls);