On 01.12.2016 16:59, Chris Wilson wrote:
> On Thu, Dec 01, 2016 at 03:06:48PM +0100, Nicolai Hähnle wrote:
>> @@ -677,15 +722,25 @@ __mutex_lock_common(struct mutex *lock, long state,
>> unsigned int subclass,
>> debug_mutex_lock_common(lock, &waiter);
>> debug_mutex_add_waiter(lock, &waiter, task);
>>
>> - /* add waiting tasks to the end of the waitqueue (FIFO): */
>> - list_add_tail(&waiter.list, &lock->wait_list);
>> + lock_contended(&lock->dep_map, ip);
>> +
>> + if (!use_ww_ctx) {
>> + /* add waiting tasks to the end of the waitqueue (FIFO): */
>> + list_add_tail(&waiter.list, &lock->wait_list);
>> + } else {
>> + /* Add in stamp order, waking up waiters that must back off. */
>> + ret = __ww_mutex_add_waiter(&waiter, lock, ww_ctx);
>> + if (ret)
>> + goto err_early_backoff;
>> +
>> + waiter.ww_ctx = ww_ctx;
>> + }
>> +
>> waiter.task = task;
>
> Would an unconditional waiter.ww_ctx = ww_ctx be chep enough? (Same
> cacheline write and all that?)
>
> Makes the above clearer in that you have
>
> if (!ww_ctx) {
> list_add_tail();
> } else {
> ret = __ww_mutex_add_waiter(); /* no need to handle !ww_ctx */
> if (ret)
> goto err_early_backoff;
> }
>
> waiter.ww_ctx = ww_ctx;
> waiter.task = task;
I don't feel strongly either way. I thought it'd be nice to have an
explicit distinction between mutex_lock(&a) and ww_mutex_lock(&a, NULL)
though.
>
>>
>> if (__mutex_waiter_is_first(lock, &waiter))
>> __mutex_set_flag(lock, MUTEX_FLAG_WAITERS);
>>
>> - lock_contended(&lock->dep_map, ip);
>> -
>> set_task_state(task, state);
>> for (;;) {
>> /*
>> @@ -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))
>
> I'm not certain about the magic of first vs HANDOFF. Afaict, first ==
> HANDOFF and this patch breaks that relationship. I think you need to add
> bool handoff; as a separate tracker to first.
>
>> goto acquired;
>>
>> /*
>> @@ -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.
>
> Comment makes sense.
>
> Ah. Should this be just if (use_ww_ctx) { /* always recheck... */ ?
> Except that !ww_ctx are never gazzumped in the list, so if they are
> first, then they are always first.
Right. See also the other mail.
Nicolai
>
> Could you explain that as well (about why !ww_ctx is special here but
> not above). And then it can even be reduced to if (ww_ctx) {} to match
> the first chunk if the revision is acceptable.
> -Chris
>
