linux-kernel - Re: WARNING in get_pi

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Date:   Tue, 31 Oct 2017 11:08:52 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     syzbot 
        <bot+2af19c9e1ffe4d4ee1d16c56ae7580feaee75765@...kaller.appspotmail.com>,
        dvhart@...radead.org, LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        syzkaller-bugs@...glegroups.com,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: WARNING in get_pi_state

On Tue, Oct 31, 2017 at 12:29:50PM +0300, Dmitry Vyukov wrote:
> I understand your sentiment, but it's definitely not _at all_. The
> system compiled this exact code, run it and triggered the bug on it.
> Do you have suggestions on how to make this code more portable? How
> does this setup would look on your system?

So I don't see the point of that tun stuff; what was is supposed to do?

All it ever did after creation was flush_tun(), which reads until empty.
But given nobody would ever write into it, that's an 'expensive' NO-OP.

> We do try hard to get rid of unnecessary stuff in reproducers. I think
> what happened in this case is the following. This is a hard to
> reproduce race. The bot was able to reproduce the crash on initial
> program that uses tun, then tried to get rid of tun code and
> re-reproduce it, but it did not reproduce this time, so it concluded
> that tun code is somehow necessary here. That's unfortunate
> consequence of testing complex concurrent code. May become somewhat
> better once we have KTSAN, the race detector.

I ripped out the tun bits and it reproduced in ~100 seconds. I've now
got it running for well over 30m on the fixed kernel while I'm trying to
come up with a comprehensible Changelog ;-)