https://github.com/torvalds/linux
Revision 1b53734bd0b2feed8e7761771b2e76fc9126ea0c authored by Roman Penyaev on 22 March 2020, 01:22:30 UTC, committed by Linus Torvalds on 22 March 2020, 01:56:06 UTC
This fixes possible lost wakeup introduced by commit a218cc491420. Originally modifications to ep->wq were serialized by ep->wq.lock, but in commit a218cc491420 ("epoll: use rwlock in order to reduce ep_poll_callback() contention") a new rw lock was introduced in order to relax fd event path, i.e. callers of ep_poll_callback() function. After the change ep_modify and ep_insert (both are called on epoll_ctl() path) were switched to ep->lock, but ep_poll (epoll_wait) was using ep->wq.lock on wqueue list modification. The bug doesn't lead to any wqueue list corruptions, because wake up path and list modifications were serialized by ep->wq.lock internally, but actual waitqueue_active() check prior wake_up() call can be reordered with modifications of ep ready list, thus wake up can be lost. And yes, can be healed by explicit smp_mb(): list_add_tail(&epi->rdlink, &ep->rdllist); smp_mb(); if (waitqueue_active(&ep->wq)) wake_up(&ep->wp); But let's make it simple, thus current patch replaces ep->wq.lock with the ep->lock for wqueue modifications, thus wake up path always observes activeness of the wqueue correcty. Fixes: a218cc491420 ("epoll: use rwlock in order to reduce ep_poll_callback() contention") Reported-by: Max Neunhoeffer <max@arangodb.com> Signed-off-by: Roman Penyaev <rpenyaev@suse.de> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Tested-by: Max Neunhoeffer <max@arangodb.com> Cc: Jakub Kicinski <kuba@kernel.org> Cc: Christopher Kohlhoff <chris.kohlhoff@clearpool.io> Cc: Davidlohr Bueso <dbueso@suse.de> Cc: Jason Baron <jbaron@akamai.com> Cc: Jes Sorensen <jes.sorensen@gmail.com> Cc: <stable@vger.kernel.org> [5.1+] Link: http://lkml.kernel.org/r/20200214170211.561524-1-rpenyaev@suse.de References: https://bugzilla.kernel.org/show_bug.cgi?id=205933 Bisected-by: Max Neunhoeffer <max@arangodb.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 parent 12e967f
Tip revision: 1b53734bd0b2feed8e7761771b2e76fc9126ea0c authored by Roman Penyaev on 22 March 2020, 01:22:30 UTC
epoll: fix possible lost wakeup on epoll_ctl() path
epoll: fix possible lost wakeup on epoll_ctl() path
Tip revision: 1b53734
File | Mode | Size |
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boot | ||
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include | ||
kernel | ||
lib | ||
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Kconfig | -rw-r--r-- | 4.6 KB |
Kconfig.debug | -rw-r--r-- | 68 bytes |
Makefile | -rw-r--r-- | 1.4 KB |
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