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Date:   Sun, 13 Oct 2019 23:29:19 -0700
From:   Davidlohr Bueso <dave@stgolabs.net>
To:     Manfred Spraul <manfred@colorfullife.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Waiman Long <longman@redhat.com>, 1vier1@web.de,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Jonathan Corbet <corbet@lwn.net>
Subject: Re: [PATCH 2/5] ipc/mqueue.c: Update/document memory barriers
Message-ID: <20191014062919.rr56mj5uzyb7sj6r@linux-p48b>
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 <20191011165527.bsdiw6gu2sk7yrnl@linux-p48b>
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On Fri, 11 Oct 2019, Manfred Spraul wrote:
>But you are right, there are two different scenarios:
>
>1) thread already in another wake_q, wakeup happens immediately after 
>the cmpxchg_relaxed().
>
>This scenario is safe, due to the smp_mb__before_atomic() in wake_q_add()
>
>2) thread woken up but e.g. a timeout, see ->state=STATE_READY, 
>returns to user space, calls sys_exit.
>
>This must not happen before get_task_struct acquired a reference.
>
>And this appears to be unsafe: get_task_struct() is refcount_inc(), 
>which is refcount_inc_checked(), which is according to lib/refcount.c 
>fully unordered.
>
>Thus: ->state=STATE_READY can execute before the refcount increase.
>
>Thus: ->state=STATE_READY needs a smp_store_release(), correct?

What if we did the reference count explicitly, and then just use
wake_q_add_safe()? That would avoid the extra barrier, __pipelined_op()
would become:

      list_del();
      get_task_struct();
      wake_q_add_safe();
      WRITE_ONCE(->state, STATE_READY);

Thanks,
Davidlohr