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From:   Marco Elver <elver@google.com>
Date:   Tue, 28 Jan 2020 09:18:38 +0100
Message-ID: <CANpmjNMzvcrQWpGWVgNRxvZroecAEZYYa2yYAtm5+ekcK=H3OQ@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>
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On Tue, 28 Jan 2020 at 04:13, Qian Cai <cai@lca.pw> wrote:
>
> > On Jan 23, 2020, at 4:36 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Wed, Jan 22, 2020 at 11:38:51PM +0100, Marco Elver wrote:
> >
> >> If possible, decode and get the line numbers. I have observed a data
> >> race in osq_lock before, however, this is the only one I have recently
> >> seen in osq_lock:
> >>
> >> read to 0xffff88812c12d3d4 of 4 bytes by task 23304 on cpu 0:
> >>  osq_lock+0x170/0x2f0 kernel/locking/osq_lock.c:143
> >>
> >>      while (!READ_ONCE(node->locked)) {
> >>              /*
> >>               * If we need to reschedule bail... so we can block.
> >>               * Use vcpu_is_preempted() to avoid waiting for a preempted
> >>               * lock holder:
> >>               */
> >> -->          if (need_resched() || vcpu_is_preempted(node_cpu(node->prev)))
> >>                      goto unqueue;
> >>
> >>              cpu_relax();
> >>      }
> >>
> >> where
> >>
> >>      static inline int node_cpu(struct optimistic_spin_node *node)
> >>      {
> >> -->          return node->cpu - 1;
> >>      }
> >>
> >>
> >> write to 0xffff88812c12d3d4 of 4 bytes by task 23334 on cpu 1:
> >> osq_lock+0x89/0x2f0 kernel/locking/osq_lock.c:99
> >>
> >>      bool osq_lock(struct optimistic_spin_queue *lock)
> >>      {
> >>              struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
> >>              struct optimistic_spin_node *prev, *next;
> >>              int curr = encode_cpu(smp_processor_id());
> >>              int old;
> >>
> >>              node->locked = 0;
> >>              node->next = NULL;
> >> -->          node->cpu = curr;
> >>
> >
> > Yeah, that's impossible. This store happens before the node is
> > published, so no matter how the load in node_cpu() is shattered, it must
> > observe the right value.
>
> Marco, any thought on how to do something about this? The worry is that
> too many false positives like this will render the tool usefulness as a
> general debug option.

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.

I will reply separately to your other email about the other data race.

Thanks,
-- Marco