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From: Alex =?utf-8?Q?Benn=C3=A9e?= <alex.bennee@linaro.org>
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Date: Fri, 24 Feb 2017 18:08:29 +0000
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Subject: Re: [Qemu-devel] Hang in aio/multi/mutex/mcs
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel <qemu-devel@nongnu.org>


Paolo Bonzini <pbonzini@redhat.com> writes:

> On 23/02/2017 20:48, Alex Bennée wrote:
>> Hope that helps the debugging ;-)
>
> Worst case we can just remove the test (it's only there for performance
> comparison, not a bug in actual QEMU code), but this seems to help here:
>
> diff --git a/tests/test-aio-multithread.c b/tests/test-aio-multithread.c
> index f11e990..8b0b40e 100644
> --- a/tests/test-aio-multithread.c
> +++ b/tests/test-aio-multithread.c
> @@ -309,7 +309,7 @@ static void mcs_mutex_lock(void)
>  static void mcs_mutex_unlock(void)
>  {
>      int next;
> -    if (nodes[id].next == -1) {
> +    if (atomic_read(&nodes[id].next) == -1) {
>          if (atomic_read(&mutex_head) == id &&
>              atomic_cmpxchg(&mutex_head, id, -1) == id) {
>              /* Last item in the list, exit.  */
> @@ -323,7 +323,7 @@ static void mcs_mutex_unlock(void)
>      }
>
>      /* Wake up the next in line.  */
> -    next = nodes[id].next;
> +    next = atomic_read(&nodes[id].next);
>      nodes[next].locked = 0;
>      qemu_futex_wake(&nodes[next].locked, 1);
>  }

Well it certainly looks like it improves the results on Travis.

Tested-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

--
Alex Bennée