From 4c44c136f2fa1378cc80dd49d1ee6501e6f2e740 Mon Sep 17 00:00:00 2001 From: Davidlohr Bueso Date: Thu, 6 May 2021 18:04:07 -0700 Subject: [PATCH] fs/epoll: restore waking from ep_done_scan() commit 7fab29e356309ff93a4b30ecc466129682ec190b upstream. Commit 339ddb53d373 ("fs/epoll: remove unnecessary wakeups of nested epoll") changed the userspace visible behavior of exclusive waiters blocked on a common epoll descriptor upon a single event becoming ready. Previously, all tasks doing epoll_wait would awake, and now only one is awoken, potentially causing missed wakeups on applications that rely on this behavior, such as Apache Qpid. While the aforementioned commit aims at having only a wakeup single path in ep_poll_callback (with the exceptions of epoll_ctl cases), we need to restore the wakeup in what was the old ep_scan_ready_list() such that the next thread can be awoken, in a cascading style, after the waker's corresponding ep_send_events(). Link: https://lkml.kernel.org/r/20210405231025.33829-3-dave@stgolabs.net Fixes: 339ddb53d373 ("fs/epoll: remove unnecessary wakeups of nested epoll") Signed-off-by: Davidlohr Bueso Cc: Al Viro Cc: Jason Baron Cc: Roman Penyaev Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- fs/eventpoll.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/fs/eventpoll.c b/fs/eventpoll.c index 2d5b172f490e..6094b2e9058b 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -746,6 +746,12 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep, */ list_splice(&txlist, &ep->rdllist); __pm_relax(ep->ws); + + if (!list_empty(&ep->rdllist)) { + if (waitqueue_active(&ep->wq)) + wake_up(&ep->wq); + } + write_unlock_irq(&ep->lock); if (!ep_locked)