Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier

Boqun Feng
2016-06-15T08:04:46Z

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Hi Waiman,

On Tue, Jun 14, 2016 at 06:48:04PM -0400, Waiman Long wrote:
> The osq_lock() and osq_unlock() function may not provide the necessary
> acquire and release barrier in some cases. This patch makes sure
> that the proper barriers are provided when osq_lock() is successful
> or when osq_unlock() is called.
>
> Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
> ---
> kernel/locking/osq_lock.c | 4 ++--
> 1 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index 05a3785..7dd4ee5 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -115,7 +115,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
> * cmpxchg in an attempt to undo our queueing.
> */
>
> - while (!READ_ONCE(node->locked)) {
> + while (!smp_load_acquire(&node->locked)) {
> /*
> * If we need to reschedule bail... so we can block.
> */
> @@ -198,7 +198,7 @@ void osq_unlock(struct optimistic_spin_queue *lock)
> * Second most likely case.
> */
> node = this_cpu_ptr(&osq_node);
> - next = xchg(&node->next, NULL);
> + next = xchg_release(&node->next, NULL);
> if (next) {
> WRITE_ONCE(next->locked, 1);

So we still use WRITE_ONCE() rather than smp_store_release() here?

Though, IIUC, This is fine for all the archs but ARM64, because there
will always be a xchg_release()/xchg() before the WRITE_ONCE(), which
carries a necessary barrier to upgrade WRITE_ONCE() to a RELEASE.

Not sure whether it's a problem on ARM64, but I think we certainly need
to add some comments here, if we count on this trick.

Am I missing something or misunderstanding you here?

Regards,
Boqun

> return;
> --
> 1.7.1
>

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