Re: [PATCH v2 05/11] locking/ww_mutex: Add waiters in stamp order

["Nicolai Hähnle" <nhaehnle@gmail.com>]

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
>