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Date:   Thu, 10 Sep 2020 12:54:58 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Sergey Nikitin <nikitins@oktetlabs.ru>
Cc:     linux-fsdevel@vger.kernel.org
Subject: Re: PROBLEM: epoll_wait() does not return events when running in
 multiple threads
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References: <8076083c-0ae3-9cef-6238-9a651b026ade@oktetlabs.ru>
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On Thu, Sep 10, 2020 at 12:48:34PM +0300, Sergey Nikitin wrote:
> Hi!
> 
> epoll does not report an event to all the threads running epoll_wait() on
> the same epoll descriptor.
> The behavior appeared in recent kernel versions starting with 5.6 probably.
> 
> How to reproduce:
> - create a pair of sockets
> - create epoll instance
> - register the socket on the epoll instance, listen for EPOLLIN events
> - start 2 threads running epoll_wait()
> - send some data to the socket
> - see that epoll_wait() within one of the threads reported an event, unlike
> another.

Could you reproduce it on mainline kernel and try to bisect it?