linux-kernel - Re: [PATCH 2/2] fs/epoll: restore waking from ep_done

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Message-Id: <20210405185018.40d437d392863f743131fcda@linux-foundation.org>
Date:   Mon, 5 Apr 2021 18:50:18 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     jbaron@...mai.com, rpenyaev@...e.de, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 2/2] fs/epoll: restore waking from ep_done_scan()

On Mon,  5 Apr 2021 16:10:25 -0700 Davidlohr Bueso <dave@...olabs.net> 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
risks.