linux-kernel - Re: BUG: MAX_STACK_TRACE

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Message-ID: <cdfeb3f8-8dc5-aa60-2782-7b3c5110edf5@acm.org>
Date:   Wed, 10 Jul 2019 12:09:34 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        linux-kernel@...r.kernel.org,
        syzbot <syzbot+6f39a9deb697359fe520@...kaller.appspotmail.com>,
        syzkaller-bugs@...glegroups.com
Subject: Re: BUG: MAX_STACK_TRACE_ENTRIES too low! (2)

On 7/10/19 11:44 AM, Eric Dumazet wrote:
> If anything using workqueues in dynamically allocated objects can turn off lockdep,
> we have a serious issue.

As far as I know that issue is only hit by syzbot tests. Anyway, I see
two possible paths forward:
* Revert the patch that makes workqueues use dynamic lockdep keys and
thereby reintroduce the false positives that lockdep reports if
different workqueues share a lockdep key. I think there is agreement
that having to analyze lockdep false positives is annoying, time
consuming and something nobody likes.
* Modify lockdep such that space in its fixed size arrays that is no
longer in use gets reused. Since the stack traces saved in the
stack_trace[] array have a variable size that array will have to be
compacted to avoid fragmentation.

Did I overlook anything?

Bart.