Re: [PATCH 2/2] fs/epoll: restore waking from ep_done_scan()

[Andrew Morton]

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