Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Mon, 5 Apr 2021 18:50:18 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Davidlohr Bueso <dave@stgolabs.net>
Cc:     jbaron@akamai.com, rpenyaev@suse.de, viro@zeniv.linux.org.uk,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Davidlohr Bueso <dbueso@suse.de>
Subject: Re: [PATCH 2/2] fs/epoll: restore waking from ep_done_scan()
Message-Id: <20210405185018.40d437d392863f743131fcda@linux-foundation.org>
In-Reply-To: <20210405231025.33829-3-dave@stgolabs.net>
References: <20210405231025.33829-1-dave@stgolabs.net>
        <20210405231025.33829-3-dave@stgolabs.net>
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Precedence: bulk

On Mon,  5 Apr 2021 16:10:25 -0700 Davidlohr Bueso <dave@stgolabs.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.