linux-kernel - Re: pselect/etc semantics

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Date:   Thu, 30 May 2019 21:03:18 +0000
From:   Eric Wong <e@...24.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Deepa Dinamani <deepa.kernel@...il.com>,
        Al Viro <viro@...IV.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        arnd@...db.de, dbueso@...e.de, axboe@...nel.dk, dave@...olabs.net,
        jbaron@...mai.com, linux-fsdevel@...r.kernel.org,
        linux-aio@...ck.org, omar.kilani@...il.com, tglx@...utronix.de,
        stable@...r.kernel.org
Subject: Re: pselect/etc semantics

"Eric W. Biederman" <ebiederm@...ssion.com> wrote:
> Frankly the only reason this appears to be worth touching is that we
> have a userspace regression.  Otherwise I would call the current
> behavior more correct and desirable than ignoring the signal longer.
> 
> If I am reading sitaution properly I suggest we go back to resoring the
> sigmask by hand in epoll_pwait, and put in a big fat comment about a
> silly userspace program depending on that behavior.
> 
> That will be easier to backport and it will just fix the regression and
> not overfix the problem for reasonable applications.

Fwiw, the cmogstored (userspace program which regressed) only
hit this regression in an obscure code path for tuning; that
code path probably sees no use outside of the test suite.

Add to that, cmogstored is an unofficial and optional component
to the obscure, largely-forgotten MogileFS.

Finally, the main users of cmogstored are on FreeBSD.  They
hit the kqueue+self-pipe code path instead of epoll_pwait.

I only used epoll_pwait on Linux since I figured I could save a
few bytes of memory by skipping eventfd/self-pipe...

Anyways, I updated cmogstored a few weeks back to call `note_run'
(the signal dispatcher) if epoll_pwait (wrapped by `mog_idleq_wait_intr')
returns 0 instead of -1 (EINTR) to workaround this kernel change:

https://bogomips.org/cmogstored-public/20190511075630.17811-1-e@80x24.org/

I could easily make a change to call `note_run' unconditionally
when `mog_idleq_wait_intr' returns, too.

But that said, there could be other users hitting the same
problem I did.  So maybe cmogstored's primary use on Linux these
days is finding regressions :>