Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <CANpmjNMzvcrQWpGWVgNRxvZroecAEZYYa2yYAtm5+ekcK=H3OQ@mail.gmail.com>
 <E65D0BFC-719A-4CF9-A934-55ACFF663F98@lca.pw>
In-Reply-To: <E65D0BFC-719A-4CF9-A934-55ACFF663F98@lca.pw>
From:   Marco Elver <elver@google.com>
Date:   Tue, 28 Jan 2020 11:29:11 +0100
Message-ID: <CANpmjNNFBLB4iB7gj3sR9y1RKB6PmneNZmfpyJ4418impvwqBA@mail.gmail.com>
Subject: Re: [PATCH] locking/osq_lock: fix a data race in osq_wait_next
To:     Qian Cai <cai@lca.pw>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>, Ingo Molnar <mingo@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "paul E. McKenney" <paulmck@kernel.org>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, 28 Jan 2020 at 11:10, Qian Cai <cai@lca.pw> wrote:
>
>
>
> > On Jan 28, 2020, at 3:18 AM, Marco Elver <elver@google.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=3Dy here and saw somet=
hing similar a splat. I=E2=80=99ll 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.