On Mon, 27 Feb 2017 12:17:26 +0100 Paolo Bonzini <pbonz...@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!
>
> Reported-by: Alex Bennée <alex.ben...@linaro.org>
> Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
> ---
Cool ! I can use travis again :)
Tested-by: Greg Kurz <gr...@kaod.org>
> tests/test-aio-multithread.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> 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);
> }
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