linux-kernel - Re: [syzbot] INFO: rcu detected stall in newstat

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Message-ID: <YaTq1I82HEA/e6r6@hirez.programming.kicks-ass.net>
Date:   Mon, 29 Nov 2021 15:59:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Hillf Danton <hdanton@...a.com>, Boqun Feng <boqun.feng@...il.com>,
        syzbot <syzbot+84ef57449019b1be878d@...kaller.appspotmail.com>,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        tglx@...utronix.de, Alan Stern <stern@...land.harvard.edu>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        rajatasthana4@...il.com
Subject: Re: [syzbot] INFO: rcu detected stall in newstat

On Mon, Nov 29, 2021 at 03:15:16PM +0100, Dmitry Vyukov wrote:

> Right, I missed the "preempt leak: 00000100 -> 00000101" warning. And
> before that there is also "WARNING: inconsistent lock state" warning.
> This reminds me of the issues we had with RCU/LOCKDEP before when an
> RCU warning disabled LOCKDEP tracking, as the result LOCKDEP missed
> part of events (e.g. tracked lock, but missed subsequent unlock) and
> due to races/ordering issues it mis-reported them as nonsensical
> reports.

You're talking about how debug_locks_off() is a hot-racy-mess? That only
matters if you're triggering stuff concurrently which *mostly* doesn't
happen.

I'm also not quite sure how to fix that without globally serializing
everything, which would be super unhappy.