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

[Thomas Hellstrom]

On 06/14/2018 01:36 PM, Peter Zijlstra wrote:
> On Thu, Jun 14, 2018 at 09:29:21AM +0200, Thomas Hellstrom wrote:
>
>>   __ww_mutex_wakeup_for_backoff(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
>>   {
>>   	struct mutex_waiter *cur;
>> +	unsigned int is_wait_die = ww_ctx->ww_class->is_wait_die;
>>   
>>   	lockdep_assert_held(&lock->wait_lock);
>>   
>> @@ -310,13 +348,14 @@ __ww_mutex_wakeup_for_backoff(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
>>   		if (!cur->ww_ctx)
>>   			continue;
>>   
>> -		if (cur->ww_ctx->acquired > 0 &&
>> +		if (is_wait_die && cur->ww_ctx->acquired > 0 &&
>>   		    __ww_ctx_stamp_after(cur->ww_ctx, ww_ctx)) {
>>   			debug_mutex_wake_waiter(lock, cur);
>>   			wake_up_process(cur->task);
>>   		}
>>   
>> -		break;
>> +		if (is_wait_die || __ww_mutex_wound(lock, cur->ww_ctx, ww_ctx))
>> +			break;
>>   	}
>>   }
> I ended up with:
>
>
> static void __sched
> __ww_mutex_check_waiters(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
> {
> 	bool is_wait_die = ww_ctx->ww_class->is_wait_die;
> 	struct mutex_waiter *cur;
>
> 	lockdep_assert_held(&lock->wait_lock);
>
> 	list_for_each_entry(cur, &lock->wait_list, list) {
> 		if (!cur->ww_ctx)
> 			continue;
>
> 		if (is_wait_die) {
> 			/*
> 			 * Because __ww_mutex_add_waiter() and
> 			 * __ww_mutex_check_stamp() wake any but the earliest
> 			 * context, this can only affect the first waiter (with
> 			 * a context).
> 			 */
> 			if (cur->ww_ctx->acquired > 0 &&
> 			    __ww_ctx_stamp_after(cur->ww_ctx, ww_ctx)) {
> 				debug_mutex_wake_waiter(lock, cur);
> 				wake_up_process(cur->task);
> 			}
>
> 			break;
> 		}
>
> 		if (__ww_mutex_wound(lock, cur->ww_ctx, ww_ctx))
> 			break;
> 	}
> }

Looks OK to me.

>
> Currently you don't allow mixing WD and WW contexts (which is not
> immediately obvious from the above code), and the above hard relies on
> that. Are there sensible use cases for mixing them? IOW will your
> current restriction stand without hassle?

Contexts _must_ agree on the algorithm used to resolve deadlocks. With 
Wait-Die, for example, older transactions will wait if a lock is held by 
a younger transaction and with Wound-Wait, younger transactions will 
wait if a lock is held by an older transaction so there is no way of 
mixing them.

Thanks,

/Thomas