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Date: Fri, 18 Sep 2015 16:41:15 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Will Deacon <will.deacon@arm.com>,
        "ebiederm@xmission.com" <ebiederm@xmission.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
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        Andrey Konovalov <andreyknvl@google.com>,
        Alexander Potapenko <glider@google.com>,
        Hans Boehm <hboehm@google.com>
Subject: Re: [PATCH] kernel: fix data race in put_pid
Message-ID: <20150918144115.GA14155@redhat.com>
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On 09/18, Dmitry Vyukov wrote:
>
> On Fri, Sep 18, 2015 at 3:44 PM, Oleg Nesterov <oleg@redhat.com> wrote:
> > So I assume that if we have
> >
> >         int X = 0;
> >         atomic_t Y = ATOMIC_INIT(0);
> >
> >         void w(void)
> >         {
> >                 X = 1;
> >                 atomic_inc_return(&Y);
> >         }
> >
> > then
> >
> >         void r(void)
> >         {
> >                 if (atomic_read_ctrl(&Y))
> >                         BUG_ON(X == 0);
> >         }
> >
> > should be correct?  Why?
> >
> > If not then I am even more confused.
>
> This not correct,

Good. because I wasn't able to understand why this could work.

> // thread 1
>                  X = 1;
>                  atomic_inc_return(&Y);
>
> // thread 2
>                  if (atomic_read_ctrl(&Y)) {
>                          X = 2;
>                          BUG_ON(X == 2);
>                   }

Thanks. This makes perfect sense to me.

And then I agree, atomic_read_ctrl() in put_pid() should fix the
theoretical problem.

Perhaps we can add this example to memory-barriers.txt... Although
perhaps it already explains/documents this case. I am afraid to open
it, it is huge and changes too often so every time it looks like a
new document to me ;)

Oleg.