Re: [PATCH] locking/mutex: Avoid spinner vs waiter starvation

[Davidlohr Bueso <dave@stgolabs.net>]

On Mon, 01 Feb 2016, Peter Zijlstra wrote:

>Subject: locking/mutex: Avoid spinner vs waiter starvation
>From: Peter Zijlstra <peterz@infradead.org>
>Date: Fri, 22 Jan 2016 12:06:53 +0100
>
>Ding Tianhong reported that under his load the optimistic spinners
>would totally starve a task that ended up on the wait list.
>
>Fix this by ensuring the top waiter also partakes in the optimistic
>spin queue.
>
>There are a few subtle differences between the assumed state of
>regular optimistic spinners and those already on the wait list, which
>result in the @acquired complication of the acquire path.
>
>Most notable are:
>
> - waiters are on the wait list and need to be taken off
> - mutex_optimistic_spin() sets the lock->count to 0 on acquire
>   even though there might be more tasks on the wait list.

Right, the main impact I see with these complications are that the
window of when a waiter takes the lock via spinning and then acquires
the wait_lock to remove itself from the list, will allow an unlock
thread to set the lock as available in the fastpath which could in
turn allow a third thread the steal the lock. With high contention,
this window will be come obviously larger as we contend for the
wait_lock.

CPU-0	      	 	       	    CPU-1			CPU-3
__mutex_lock_common		    
   mutex_optimistic_spin
   (->count now 0)
			__mutex_fastpath_unlock
			(->count now 1)				 __mutex_fastpath_lock
				     				 (stolen)
														
spin_lock_mutex(&lock->wait_lock, flags);

But we've always been bad when it comes to counter and waiters.

Thanks,
Davidlohr