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From: Andrew Morton <>
To: Davidlohr Bueso <>
	Davidlohr Bueso <>
Subject: Re: [PATCH 2/2] fs/epoll: restore waking from ep_done_scan()
Date: Mon, 5 Apr 2021 18:50:18 -0700	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Mon,  5 Apr 2021 16:10:25 -0700 Davidlohr Bueso <> wrote:

> 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().

Tricky.  339ddb53d373 was merged in December 2019.  So do we backport
this fix?  Could any userspace code be depending upon the
post-339ddb53d373 behaviour?

Or do we just leave the post-339ddb53d373 code as-is?  Presumably the
issue is very rarely encountered, and changeing it back has its own

  reply	other threads:[~2021-04-06  1:50 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-05 23:10 [PATCH 0/2] fs/epoll: restore user-visible behavior upon event ready Davidlohr Bueso
2021-04-05 23:10 ` [PATCH 1/2] kselftest: introduce new epoll test case Davidlohr Bueso
2021-04-05 23:10 ` [PATCH 2/2] fs/epoll: restore waking from ep_done_scan() Davidlohr Bueso
2021-04-06  1:50   ` Andrew Morton [this message]
2021-04-06  3:22     ` Davidlohr Bueso
2021-04-06  5:09       ` Andrew Morton

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