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Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list is not NULL.

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On Fri, Jan 22, 2016 at 02:20:19AM -0800, Jason Low wrote:

> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -543,6 +543,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>  	lock_contended(&lock->dep_map, ip);
>  
>  	for (;;) {
> +		bool acquired = false;
> +
>  		/*
>  		 * Lets try to take the lock again - this is needed even if
>  		 * we get here for the first time (shortly after failing to
> @@ -577,7 +579,12 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>  		/* didn't get the lock, go to sleep: */
>  		spin_unlock_mutex(&lock->wait_lock, flags);
>  		schedule_preempt_disabled();
> +
> +		if (mutex_is_locked(lock))
> +			acquired = mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx);
>  		spin_lock_mutex(&lock->wait_lock, flags);
> +		if (acquired)
> +			break;
>  	}
>  	__set_task_state(task, TASK_RUNNING);

I think the problem here is that mutex_optimistic_spin() leaves the
mutex->count == 0, even though we have waiters (us at the very least).

But this should be easily fixed, since if we acquired, we should be the
one releasing, so there's no race.

So something like so:

		if (acquired) {
			atomic_set(&mutex->count, -1);
			break;
		}

Should deal with that -- we'll set it to 0 again a little further down
if the list ends up empty.


There might be other details, but this is the one that stood out.

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Thread

[PATCH RFC] locking/mutexes: don't spin on owner when wait list is  not NULL. Ding Tianhong <dingtianhong@huawei.com> - 2016-01-21 10:40 +0100
  Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Tim Chen <tim.c.chen@linux.intel.com> - 2016-01-21 22:30 +0100
    Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Davidlohr Bueso <dave@stgolabs.net> - 2016-01-22 03:50 +0100
      Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-01-22 04:20 +0100
    Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-01-22 03:50 +0100
  Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Waiman Long <waiman.long@hpe.com> - 2016-01-22 00:10 +0100
    Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Davidlohr Bueso <dave@stgolabs.net> - 2016-01-22 07:20 +0100
      Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Waiman Long <waiman.long@hpe.com> - 2016-01-22 14:40 +0100
        Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Davidlohr Bueso <dave@stgolabs.net> - 2016-01-22 17:50 +0100
          [PATCH] locking/mutex: Allow next waiter lockless wakeup Davidlohr Bueso <dave@stgolabs.net> - 2016-01-25 03:30 +0100
            Re: [PATCH] locking/mutex: Allow next waiter lockless wakeup Waiman Long <waiman.long@hpe.com> - 2016-01-26 00:10 +0100
    Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Peter Zijlstra <peterz@infradead.org> - 2016-01-22 10:00 +0100
      Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Jason Low <jason.low2@hp.com> - 2016-01-22 11:30 +0100
        Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Peter Zijlstra <peterz@infradead.org> - 2016-01-22 12:00 +0100
          Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Peter Zijlstra <peterz@infradead.org> - 2016-01-22 12:00 +0100
            Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Peter Zijlstra <peterz@infradead.org> - 2016-01-22 12:10 +0100
              Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Waiman Long <waiman.long@hpe.com> - 2016-01-22 15:10 +0100
                Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Ding Tianhong <dingtianhong@huawei.com> - 2016-01-24 09:10 +0100
      Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list  is not NULL. Waiman Long <waiman.long@hpe.com> - 2016-01-22 14:50 +0100

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