linux-kernel - Re: [patch] fs, epoll: short circuit fetching events if thread has been killed

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Message-ID: <alpine.DEB.2.10.1705091707340.112797@chino.kir.corp.google.com>
Date:   Tue, 9 May 2017 17:08:17 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-kernel@...r.kernel.org, Jan Kara <jack@...e.cz>,
        Davide Libenzi <davidel@...ilserver.org>
Subject: Re: [patch] fs, epoll: short circuit fetching events if thread has
 been killed

On Tue, 9 May 2017, Andrew Morton wrote:

> > We've encountered zombies that are waiting for a thread to exit that are
> > looping in ep_poll() almost endlessly although there is a pending SIGKILL
> > as a result of a group exit.
> > 
> > This happens because we always find ep_events_available() and fetch more
> > events and never are able to check for signal_pending() that would break
> > from the loop and return -EINTR.
> > 
> > Special case fatal signals and break immediately to guarantee that we
> > loop to fetch more events and delay making a timely exit.
> > 
> > It would also be possible to simply move the check for signal_pending()
> > higher than checking for ep_events_available(), but there have been no
> > reports of delayed signal handling other than SIGKILL preventing zombies
> > from exiting that would be fixed by this.
> 
> Any thoughts on the priority of this?  -stable?  If so, why?
> 

It fixes an issue for us where we have witnessed zombies sticking around 
for at least O(minutes), but considering the code has been like this 
forever and nobody else has complained that I have found, I would simply 
queue it up for 4.12.