linux-kernel - Re: [PATCH] locking/osq_lock: fix a data race in osq_wait

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Message-ID: <CANpmjNNFBLB4iB7gj3sR9y1RKB6PmneNZmfpyJ4418impvwqBA@mail.gmail.com>
Date:   Tue, 28 Jan 2020 11:29:11 +0100
From:   Marco Elver <elver@...gle.com>
To:     Qian Cai <cai@....pw>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH] locking/osq_lock: fix a data race in osq_wait_next

On Tue, 28 Jan 2020 at 11:10, Qian Cai <cai@....pw> wrote:
>
>
>
> > On Jan 28, 2020, at 3:18 AM, Marco Elver <elver@...gle.com> wrote:
> >
> > This should be an instance of same-value-store, since the node->cpu is
> > per-CPU and smp_processor_id() should always be the same, at least
> > once it's published. I believe the data race I observed here before
> > KCSAN had KCSAN_REPORT_VALUE_CHANGE_ONLY on syzbot, and hasn't been
> > observed since. For the most part, that should deal with this case.
>
> Are you sure? I had KCSAN_REPORT_VALUE_CHANGE_ONLY=y here and saw something similar a splat. I’ll also double check on my side and provide the decoding.

The data race you reported in this thread is a different one (same
function, but different accesses). I will reply separately.