From: Soheil Hassas Yeganeh soheil@google.com
Commit 289caf5d8f6c61c6d2b7fd752a7f483cd153f182 upstream.
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-gq1kjD...
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 abc610e01c66 ("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 abc610e01c66, it is guaranteed that Thread 3 will wake up either by Thread 1 or Thread 2. After abc610e01c66, 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-gq1kjD... Link: https://lkml.kernel.org/r/20201106231635.3528496-2-soheil.kdev@gmail.com Fixes: abc610e01c66 ("fs/epoll: avoid barrier after an epoll_wait(2) timeout") Signed-off-by: Soheil Hassas Yeganeh soheil@google.com Tested-by: Guantao Liu guantaol@google.com Suggested-by: Linus Torvalds torvalds@linux-foundation.org Reported-by: Guantao Liu guantaol@google.com Reviewed-by: Eric Dumazet edumazet@google.com Reviewed-by: Willem de Bruijn willemb@google.com Reviewed-by: Khazhismel Kumykov khazhy@google.com Reviewed-by: Davidlohr Bueso dbueso@suse.de Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Rishabh Bhatnagar risbhat@amazon.com --- fs/eventpoll.c | 25 ++++++++++++++++--------- 1 file changed, 16 insertions(+), 9 deletions(-)
diff --git a/fs/eventpoll.c b/fs/eventpoll.c index e5496483a882..877f9f61a4e8 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -1928,23 +1928,30 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events, } write_unlock_irq(&ep->lock);
- if (eavail || res) - break; + if (!eavail && !res) + timed_out = !schedule_hrtimeout_range(to, slack, + HRTIMER_MODE_ABS);
- if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS)) { - timed_out = 1; - break; - } - - /* We were woken up, thus go and try to harvest some events */ + /* + * We were woken up, thus go and try to harvest some events. + * If timed out and still on the wait queue, recheck eavail + * carefully under lock, below. + */ eavail = 1; - } while (0);
__set_current_state(TASK_RUNNING);
if (!list_empty_careful(&wait.entry)) { 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); write_unlock_irq(&ep->lock); }