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

[Waiman Long]

On 02/02/2016 04:19 PM, Davidlohr Bueso wrote:
> On Mon, 01 Feb 2016, Peter Zijlstra wrote:
>
> > Subject: locking/mutex: Avoid spinner vs waiter starvation
> > From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > 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

I don't quite get how that can happen. CPU0 cannot change the count to 0 
unless CPU1, the lock holder, does the unlock first. Once CPU0 sees a 
count of 1 and change it to 0, it is the lock holder and there can be no 
other CPU that can do the unlock.

Cheers,
Longman