[PATCH v2 1/2] locking: Implement an algorithm choice for Wound-Wait mutexes

[Peter Zijlstra]

On Thu, Jun 14, 2018 at 09:29:21AM +0200, Thomas Hellstrom wrote:
> +static bool __ww_mutex_wound(struct mutex *lock,
> +			     struct ww_acquire_ctx *ww_ctx,
> +			     struct ww_acquire_ctx *hold_ctx)
> +{
> +	struct task_struct *owner = __mutex_owner(lock);
> +
> +	lockdep_assert_held(&lock->wait_lock);
> +
> +	if (owner && hold_ctx && __ww_ctx_stamp_after(hold_ctx, ww_ctx) &&
> +	    ww_ctx->acquired > 0) {
> +		hold_ctx->wounded = 1;
> +
> +		/*
> +		 * wake_up_process() paired with set_current_state() inserts
> +		 * sufficient barriers to make sure @owner either sees it's
> +		 * wounded or has a wakeup pending to re-read the wounded
> +		 * state.
> +		 *
> +		 * The value of hold_ctx->wounded in
> +		 * __ww_mutex_lock_check_stamp();
> +		 */
> +		if (owner != current)
> +			wake_up_process(owner);
> +
> +		return true;
> +	}
> +
> +	return false;
> +}

> @@ -338,12 +377,18 @@ ww_mutex_set_context_fastpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
>  	 * and keep spinning, or it will acquire wait_lock, add itself
>  	 * to waiter list and sleep.
>  	 */
> -	smp_mb(); /* ^^^ */
> +	smp_mb(); /* See comments above and below. */
>  
>  	/*
> -	 * Check if lock is contended, if not there is nobody to wake up
> +	 * Check if lock is contended, if not there is nobody to wake up.
> +	 * We can use list_empty() unlocked here since it only compares a
> +	 * list_head field pointer to the address of the list head
> +	 * itself, similarly to how list_empty() can be considered RCU-safe.
> +	 * The memory barrier above pairs with the memory barrier in
> +	 * __ww_mutex_add_waiter and makes sure lock->ctx is visible before
> +	 * we check for waiters.
>  	 */
> -	if (likely(!(atomic_long_read(&lock->base.owner) & MUTEX_FLAG_WAITERS)))
> +	if (likely(list_empty(&lock->base.wait_list)))
>  		return;
>  

OK, so what happens is that if we see !empty list, we take wait_lock,
if we end up in __ww_mutex_wound() we must really have !empty wait-list.

It can however still see !owner because __mutex_unlock_slowpath() can
clear the owner field. But if owner is set, it must stay valid because
FLAG_WAITERS and we're holding wait_lock.

So the wake_up_process() is in fact safe.

Let me put that in a comment.