epoll: remove unnecessary xchg (d1bc90dd) · Commits · 戴 / test

xchg in ep_unregister_pollwait() is unnecessary because it is protected by either epmutex or ep->mtx (the same protection as ep_remove()). If xchg was necessary, it would be insufficient to protect against problems: if multiple concurrent calls to ep_unregister_pollwait() were possible then a second caller that returns without doing anything because nwait == 0 could return before the waitqueues are removed by the first caller, which looks like it could lead to problematic races with ep_poll_callback(). So remove xchg and add comments about the locking. Signed-off-by:

Tony Battersby <tonyb@cybernetics.com> Acked-by:

Davide Libenzi <davidel@xmailserver.org> Signed-off-by:

Andrew Morton <akpm@linux-foundation.org> Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

fs/eventpoll.c

+8 −14

Original line number	Diff line number	Diff line
		@@ -394,29 +394,23 @@ static void ep_poll_safewake(wait_queue_head_t *wq)
		}

		/*
		* This function unregister poll callbacks from the associated file descriptor.
		* Since this must be called without holding "ep->lock" the atomic exchange trick
		* will protect us from multiple unregister.
		* This function unregisters poll callbacks from the associated file
		* descriptor. Must be called with "mtx" held (or "epmutex" if called from
		* ep_free).
		*/
		static void ep_unregister_pollwait(struct eventpoll ep, struct epitem epi)
		{
		int nwait;
		struct list_head *lsthead = &epi->pwqlist;
		struct eppoll_entry *pwq;

		/* This is called without locks, so we need the atomic exchange */
		nwait = xchg(&epi->nwait, 0);

		if (nwait) {
		while (!list_empty(lsthead)) {
		pwq = list_first_entry(lsthead, struct eppoll_entry, llink);

		list_del_init(&pwq->llink);
		list_del(&pwq->llink);
		remove_wait_queue(pwq->whead, &pwq->wait);
		kmem_cache_free(pwq_cache, pwq);
		}
		}
		}

		/**
		* ep_scan_ready_list - Scans the ready list in a way that makes possible for

Admin message