Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop

[Waiman Long]

On 10/07/2016 10:52 AM, Peter Zijlstra wrote:
> Doesn't really matter yet, but pull the HANDOFF and trylock out from
> under the wait_lock.
>
> The intention is to add an optimistic spin loop here, which requires
> we do not hold the wait_lock, so shuffle code around in preparation.
>
> Also clarify the purpose of taking the wait_lock in the wait loop, its
> tempting to want to avoid it altogether, but the cancellation cases
> need to to avoid losing wakeups.
>
> Suggested-by: Waiman Long<waiman.long@xxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel)<peterz@xxxxxxxxxxxxx>
> ---
>   kernel/locking/mutex.c |   30 +++++++++++++++++++++++++-----
>   1 file changed, 25 insertions(+), 5 deletions(-)

> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -631,13 +631,21 @@ __mutex_lock_common(struct mutex *lock,
>
>   	lock_contended(&lock->dep_map, ip);
>
> +	set_task_state(task, state);

Do we want to set the state here? I am not sure if it is OK to set the 
task state without ever calling schedule().

>   	for (;;) {
> +		/*
> +		 * Once we hold wait_lock, we're serialized against
> +		 * mutex_unlock() handing the lock off to us, do a trylock
> +		 * before testing the error conditions to make sure we pick up
> +		 * the handoff.
> +		 */
>   		if (__mutex_trylock(lock, first))
> -			break;
> +			goto acquired;
>
>   		/*
> -		 * got a signal? (This code gets eliminated in the
> -		 * TASK_UNINTERRUPTIBLE case.)
> +		 * Check for signals and wound conditions while holding
> +		 * wait_lock. This ensures the lock cancellation is ordered
> +		 * against mutex_unlock() and wake-ups do not go missing.
>   		 */
>   		if (unlikely(signal_pending_state(state, task))) {
>   			ret = -EINTR;
> @@ -650,16 +658,27 @@ __mutex_lock_common(struct mutex *lock,
>   				goto err;
>   		}
>
> -		__set_task_state(task, state);
>   		spin_unlock_mutex(&lock->wait_lock, flags);
>   		schedule_preempt_disabled();
> -		spin_lock_mutex(&lock->wait_lock, flags);
>
>   		if (!first&&  __mutex_waiter_is_first(lock,&waiter)) {
>   			first = true;
>   			__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
>   		}
> +
> +		set_task_state(task, state);

I would suggest keep the __set_task_state() above and change 
set_task_state(task, state) to set_task_state(task, TASK_RUNNING) to 
provide the memory barrier. Then we don't need adding __set_task_state() 
calls below.

> +		/*
> +		 * Here we order against unlock; we must either see it change
> +		 * state back to RUNNING and fall through the next schedule(),
> +		 * or we must see its unlock and acquire.
> +		 */
> +		if (__mutex_trylock(lock, first))
> +			break;
> +

I don't think we need a trylock here since we are going to do it at the 
top of the loop within wait_lock anyway.

> +		spin_lock_mutex(&lock->wait_lock, flags);
>   	}
> +	spin_lock_mutex(&lock->wait_lock, flags);
> +acquired:
>   	__set_task_state(task, TASK_RUNNING);
>
>   	mutex_remove_waiter(lock,&waiter, task);
> @@ -682,6 +701,7 @@ __mutex_lock_common(struct mutex *lock,
>   	return 0;
>
>   err:
> +	__set_task_state(task, TASK_RUNNING);
>   	mutex_remove_waiter(lock,&waiter, task);
>   	spin_unlock_mutex(&lock->wait_lock, flags);
>   	debug_mutex_free_waiter(&waiter);

Cheers,
Longman