Re: [RFC PATCH-tip v4 01/10] locking/osq: Make lock/unlock proper acquire/release barrier

[Davidlohr Bueso]

On Thu, 18 Aug 2016, 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.

But why do we need these guarantees given that osq is only used internally
for lock owner spinning situations? Leaking out of the critical region will
obviously be bad if using it as a full lock, but, as is, this can only hurt
performance of two of the most popular locks in the kernel -- although yes,
using smp_acquire__after_ctrl_dep is nicer for polling.

If you need tighter osq for rwsems, could it be refactored such that mutexes
do not take a hit?

> Suggested-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Waiman Long <Waiman.Long@xxxxxxx>
> ---
> kernel/locking/osq_lock.c |   24 ++++++++++++++++++------
> 1 files changed, 18 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index 05a3785..3da0b97 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -124,6 +124,11 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>
> 		cpu_relax_lowlatency();
> 	}
> +	/*
> +	 * Add an acquire memory barrier for pairing with the release barrier
> +	 * in unlock.
> +	 */
> +	smp_acquire__after_ctrl_dep();
> 	return true;
>
> unqueue:
> @@ -198,13 +203,20 @@ void osq_unlock(struct optimistic_spin_queue *lock)
> 	 * Second most likely case.
> 	 */
> 	node = this_cpu_ptr(&osq_node);
> -	next = xchg(&node->next, NULL);
> -	if (next) {
> -		WRITE_ONCE(next->locked, 1);
> +	next = xchg_relaxed(&node->next, NULL);
> +	if (next)
> +		goto unlock;
> +
> +	next = osq_wait_next(lock, node, NULL);
> +	if (unlikely(!next)) {
> +		/*
> +		 * In the unlikely event that the OSQ is empty, we need to
> +		 * provide a proper release barrier.
> +		 */
> +		smp_mb();
> 		return;
> 	}
>
> -	next = osq_wait_next(lock, node, NULL);
> -	if (next)
> -		WRITE_ONCE(next->locked, 1);
> +unlock:
> +	smp_store_release(&next->locked, 1);
> }

As well as for the smp_acquire__after_ctrl_dep comment you have above, this also
obviously pairs with the osq_lock's smp_load_acquire while backing out (unqueueing,
step A). Given the above, for this case we might also just rely on READ_ONCE(node->locked),
if we get the conditional wrong and miss the node becoming locked, all we do is another
iteration, and while there is a cmpxchg() there, it is mitigated with the ccas thingy.

Thanks,
Davidlohr
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