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Date: Mon, 27 Feb 2017 13:01:20 +0100
From: Greg Kurz <groug@kaod.org>
Message-ID: <20170227130120.6ec54cc2@bahia.lan>
In-Reply-To: <20170227111726.9237-1-pbonzini@redhat.com>
References: <20170227111726.9237-1-pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] tests-aio-multithread: use atomic_read
 properly
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org, stefanha@redhat.com

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On Mon, 27 Feb 2017 12:17:26 +0100
Paolo Bonzini <pbonzini@redhat.com> wrote:

> nodes[id].next is written by other threads.  If atomic_read is not used
> (matching atomic_set in mcs_mutex_lock!) the compiler can optimize the
> whole "if" away!
>=20
> Reported-by: Alex Benn=C3=A9e <alex.bennee@linaro.org>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---

Cool ! I can use travis again :)

Tested-by: Greg Kurz <groug@kaod.org>

>  tests/test-aio-multithread.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>=20
> 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 =3D=3D -1) {
> +    if (atomic_read(&nodes[id].next) =3D=3D -1) {
>          if (atomic_read(&mutex_head) =3D=3D id &&
>              atomic_cmpxchg(&mutex_head, id, -1) =3D=3D id) {
>              /* Last item in the list, exit.  */
> @@ -323,7 +323,7 @@ static void mcs_mutex_unlock(void)
>      }
> =20
>      /* Wake up the next in line.  */
> -    next =3D nodes[id].next;
> +    next =3D atomic_read(&nodes[id].next);
>      nodes[next].locked =3D 0;
>      qemu_futex_wake(&nodes[next].locked, 1);
>  }


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