From: Davidlohr Bueso dave@stgolabs.net Subject: fs/epoll: restore waking from ep_done_scan()

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 dbueso@suse.de Cc: Al Viro viro@zeniv.linux.org.uk Cc: Jason Baron jbaron@akamai.com Cc: Roman Penyaev rpenyaev@suse.de Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

fs/eventpoll.c | 6 ++++++ 1 file changed, 6 insertions(+)

--- a/fs/eventpoll.c~fs-epoll-restore-waking-from-ep_done_scan +++ a/fs/eventpoll.c @@ -657,6 +657,12 @@ static void ep_done_scan(struct eventpol */ 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); }

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