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SubjectRe: [PATCH v2 05/11] locking/ww_mutex: Add waiters in stamp order
On Thu, Dec 01, 2016 at 03:06:48PM +0100, Nicolai Hähnle wrote:
> @@ -693,8 +748,12 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> * mutex_unlock() handing the lock off to us, do a trylock
> * before testing the error conditions to make sure we pick up
> * the handoff.
> + *
> + * For w/w locks, we always need to do this even if we're not
> + * currently the first waiter, because we may have been the
> + * first waiter during the unlock.
> */
> - if (__mutex_trylock(lock, first))
> + if (__mutex_trylock(lock, use_ww_ctx || first))
> goto acquired;

So I'm somewhat uncomfortable with this. The point is that with the
.handoff logic it is very easy to accidentally allow:

mutex_lock(&a);
mutex_lock(&a);

And I'm not sure this doesn't make that happen for ww_mutexes. We get to
this __mutex_trylock() without first having blocked.


> /*
> @@ -716,7 +775,20 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> spin_unlock_mutex(&lock->wait_lock, flags);
> schedule_preempt_disabled();
>
> - if (!first && __mutex_waiter_is_first(lock, &waiter)) {
> + if (use_ww_ctx && ww_ctx) {
> + /*
> + * Always re-check whether we're in first position. We
> + * don't want to spin if another task with a lower
> + * stamp has taken our position.
> + *
> + * We also may have to set the handoff flag again, if
> + * our position at the head was temporarily taken away.
> + */
> + first = __mutex_waiter_is_first(lock, &waiter);
> +
> + if (first)
> + __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
> + } else if (!first && __mutex_waiter_is_first(lock, &waiter)) {
> first = true;
> __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
> }

So the point is that !ww_ctx entries are 'skipped' during the insertion
and therefore, if one becomes first, it must stay first?

> @@ -728,7 +800,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> * or we must see its unlock and acquire.
> */
> if ((first && mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, true)) ||
> - __mutex_trylock(lock, first))
> + __mutex_trylock(lock, use_ww_ctx || first))
> break;
>
> spin_lock_mutex(&lock->wait_lock, flags);


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