Commit a25ddb5f authored by Soheil Hassas Yeganeh's avatar Soheil Hassas Yeganeh Committed by Stephen Rothwell
Browse files

epoll: check for events when removing a timed out thread from the wait queue

Patch series "simplify ep_poll".

This patch series is a followup based on the suggestions and feedback by
Linus:
https://lkml.kernel.org/r/CAHk-=wizk=OxUyQPbO8MS41w2Pag1kniUV5WdD5qWL-gq1kjDA@mail.gmail.com

The first patch in the series is a fix for the epoll race in presence of
timeouts, so that it can be cleanly backported to all affected stable
kernels.

The rest of the patch series simplify the ep_poll() implementation.  Some
of these simplifications result in minor performance enhancements as well.
We have kept these changes under self tests and internal benchmarks for a
few days, and there are minor (1-2%) performance enhancements as a result.

This patch (of 8):

After abc610e0 ("fs/epoll: avoid barrier after an epoll_wait(2)
timeout"), we break out of the ep_poll loop upon timeout, without checking
whether there is any new events available.  Prior to that patch-series we
always called ep_events_available() after exiting the loop.

This can cause races and missed wakeups.  For example, consider the
following scenario reported by Guantao Liu:

Suppose we have an eventfd added using EPOLLET to an epollfd.

Thread 1: Sleeps for just below 5ms and then writes to an eventfd.
Thread 2: Calls epoll_wait with a timeout of 5 ms. If it sees an
          event of the eventfd, it will write back on that fd.
Thread 3: Calls epoll_wait with a negative timeout.

Prior to abc610e0, it is guaranteed that Thread 3 will wake up either
by Thread 1 or Thread 2.  After abc610e0, Thread 3 can be blocked
indefinitely if Thread 2 sees a timeout right before the write to the
eventfd by Thread 1.  Thread 2 will be woken up from
schedule_hrtimeout_range and, with evail 0, it will not call
ep_send_events().

To fix this issue:
1) Simplify the timed_out case as suggested by Linus.
2) while holding the lock, recheck whether the thread was woken up
   after its time out has reached.

Note that (2) is different from Linus' original suggestion: It do not set
"eavail = ep_events_available(ep)" to avoid unnecessary contention (when
there are too many timed-out threads and a small number of events), as
well as races mentioned in the discussion thread.

This is the first patch in the series so that the backport to stable
releases is straightforward.

Link: https://lkml.kernel.org/r/20201106231635.3528496-1-soheil.kdev@gmail.com
Link: https://lkml.kernel.org/r/CAHk-=wizk=OxUyQPbO8MS41w2Pag1kniUV5WdD5qWL-gq1kjDA@mail.gmail.com
Link: https://lkml.kernel.org/r/20201106231635.3528496-2-soheil.kdev@gmail.com
Fixes: abc610e0

 ("fs/epoll: avoid barrier after an epoll_wait(2) timeout")
Signed-off-by: default avatarSoheil Hassas Yeganeh <soheil@google.com>
Tested-by: default avatarGuantao Liu <guantaol@google.com>
Suggested-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Reported-by: default avatarGuantao Liu <guantaol@google.com>
Reviewed-by: default avatarEric Dumazet <edumazet@google.com>
Reviewed-by: default avatarWillem de Bruijn <willemb@google.com>
Reviewed-by: default avatarKhazhismel Kumykov <khazhy@google.com>
Reviewed-by: default avatarDavidlohr Bueso <dbueso@suse.de>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarStephen Rothwell <sfr@canb.auug.org.au>
parent 30944760
...@@ -1817,23 +1817,30 @@ fetch_events: ...@@ -1817,23 +1817,30 @@ fetch_events:
} }
write_unlock_irq(&ep->lock); write_unlock_irq(&ep->lock);
if (eavail || res) if (!eavail && !res)
break; timed_out = !schedule_hrtimeout_range(to, slack,
HRTIMER_MODE_ABS);
if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS)) { /*
timed_out = 1; * We were woken up, thus go and try to harvest some events.
break; * If timed out and still on the wait queue, recheck eavail
} * carefully under lock, below.
*/
/* We were woken up, thus go and try to harvest some events */
eavail = 1; eavail = 1;
} while (0); } while (0);
__set_current_state(TASK_RUNNING); __set_current_state(TASK_RUNNING);
if (!list_empty_careful(&wait.entry)) { if (!list_empty_careful(&wait.entry)) {
write_lock_irq(&ep->lock); write_lock_irq(&ep->lock);
/*
* If the thread timed out and is not on the wait queue, it
* means that the thread was woken up after its timeout expired
* before it could reacquire the lock. Thus, when wait.entry is
* empty, it needs to harvest events.
*/
if (timed_out)
eavail = list_empty(&wait.entry);
__remove_wait_queue(&ep->wq, &wait); __remove_wait_queue(&ep->wq, &wait);
write_unlock_irq(&ep->lock); write_unlock_irq(&ep->lock);
} }
......
Supports Markdown
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment