Commit efcdd350 authored by Jason Baron's avatar Jason Baron Committed by Linus Torvalds
Browse files

fs/epoll: make nesting accounting safe for -rt kernel 



Davidlohr Bueso pointed out that when CONFIG_DEBUG_LOCK_ALLOC is set
ep_poll_safewake() can take several non-raw spinlocks after disabling
interrupts.  Since a spinlock can block in the -rt kernel, we can't take a
spinlock after disabling interrupts.  So let's re-work how we determine
the nesting level such that it plays nicely with the -rt kernel.

Let's introduce a 'nests' field in struct eventpoll that records the
current nesting level during ep_poll_callback().  Then, if we nest again
we can find the previous struct eventpoll that we were called from and
increase our count by 1.  The 'nests' field is protected by
ep->poll_wait.lock.

I've also moved the visited field to reduce the size of struct eventpoll
from 184 bytes to 176 bytes on x86_64 for !CONFIG_DEBUG_LOCK_ALLOC, which
is typical for a production config.

Reported-by: default avatarDavidlohr Bueso <dbueso@suse.de>
Signed-off-by: default avatarJason Baron <jbaron@akamai.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Reviewed-by: default avatarDavidlohr Bueso <dbueso@suse.de>
Cc: Roman Penyaev <rpenyaev@suse.de>
Cc: Eric Wong <normalperson@yhbt.net>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Link: http://lkml.kernel.org/r/1582739816-13167-1-git-send-email-jbaron@akamai.com


Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 282144e0

fs/eventpoll.c

+43 −21
Original line number Diff line number Diff line
@@ -218,13 +218,18 @@ struct eventpoll { 
	struct file *file;



	/* used to optimize loop detection check */

	int visited;

	struct list_head visited_list_link;

	int visited;



#ifdef CONFIG_NET_RX_BUSY_POLL

	/* used to track busy poll napi_id */

	unsigned int napi_id;

#endif



#ifdef CONFIG_DEBUG_LOCK_ALLOC

	/* tracks wakeup nests for lockdep validation */

	u8 nests;

#endif

};



/* Wait structure used by the poll hooks */
@@ -545,30 +550,47 @@ out_unlock: 
 */

#ifdef CONFIG_DEBUG_LOCK_ALLOC



static DEFINE_PER_CPU(int, wakeup_nest);



static void ep_poll_safewake(wait_queue_head_t *wq)

static void ep_poll_safewake(struct eventpoll *ep, struct epitem *epi)

{

	struct eventpoll *ep_src;

	unsigned long flags;

	int subclass;



	local_irq_save(flags);

	preempt_disable();

	subclass = __this_cpu_read(wakeup_nest);

	spin_lock_nested(&wq->lock, subclass + 1);

	__this_cpu_inc(wakeup_nest);

	wake_up_locked_poll(wq, POLLIN);

	__this_cpu_dec(wakeup_nest);

	spin_unlock(&wq->lock);

	local_irq_restore(flags);

	preempt_enable();

	u8 nests = 0;



	/*

	 * To set the subclass or nesting level for spin_lock_irqsave_nested()

	 * it might be natural to create a per-cpu nest count. However, since

	 * we can recurse on ep->poll_wait.lock, and a non-raw spinlock can

	 * schedule() in the -rt kernel, the per-cpu variable are no longer

	 * protected. Thus, we are introducing a per eventpoll nest field.

	 * If we are not being call from ep_poll_callback(), epi is NULL and

	 * we are at the first level of nesting, 0. Otherwise, we are being

	 * called from ep_poll_callback() and if a previous wakeup source is

	 * not an epoll file itself, we are at depth 1 since the wakeup source

	 * is depth 0. If the wakeup source is a previous epoll file in the

	 * wakeup chain then we use its nests value and record ours as

	 * nests + 1. The previous epoll file nests value is stable since its

	 * already holding its own poll_wait.lock.

	 */

	if (epi) {

		if ((is_file_epoll(epi->ffd.file))) {

			ep_src = epi->ffd.file->private_data;

			nests = ep_src->nests;

		} else {

			nests = 1;

		}

	}

	spin_lock_irqsave_nested(&ep->poll_wait.lock, flags, nests);

	ep->nests = nests + 1;

	wake_up_locked_poll(&ep->poll_wait, EPOLLIN);

	ep->nests = 0;

	spin_unlock_irqrestore(&ep->poll_wait.lock, flags);

}



#else



static void ep_poll_safewake(wait_queue_head_t *wq)

static void ep_poll_safewake(struct eventpoll *ep, struct epitem *epi)

{

	wake_up_poll(wq, EPOLLIN);

	wake_up_poll(&ep->poll_wait, EPOLLIN);

}



#endif
@@ -789,7 +811,7 @@ static void ep_free(struct eventpoll *ep) 


	/* We need to release all tasks waiting for these file */

	if (waitqueue_active(&ep->poll_wait))

		ep_poll_safewake(&ep->poll_wait);

		ep_poll_safewake(ep, NULL);



	/*

	 * We need to lock this because we could be hit by
@@ -1258,7 +1280,7 @@ out_unlock: 


	/* We have to call this outside the lock */

	if (pwake)

		ep_poll_safewake(&ep->poll_wait);

		ep_poll_safewake(ep, epi);



	if (!(epi->event.events & EPOLLEXCLUSIVE))

		ewake = 1;
@@ -1562,7 +1584,7 @@ static int ep_insert(struct eventpoll *ep, const struct epoll_event *event, 


	/* We have to call this outside the lock */

	if (pwake)

		ep_poll_safewake(&ep->poll_wait);

		ep_poll_safewake(ep, NULL);



	return 0;
@@ -1666,7 +1688,7 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi, 


	/* We have to call this outside the lock */

	if (pwake)

		ep_poll_safewake(&ep->poll_wait);

		ep_poll_safewake(ep, NULL);



	return 0;

}

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