linux-kernel - Re: [PATCH v2 1/7] locking/rwsem: check for active writer/spinner before wakeup

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Date:	Fri, 08 Aug 2014 13:21:46 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	Jason Low <jason.low2@...com>
Cc:	Waiman Long <Waiman.Long@...com>, Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@...com>,
	aswin@...com
Subject: Re: [PATCH v2 1/7] locking/rwsem: check for active writer/spinner
 before wakeup

On Fri, 2014-08-08 at 12:50 -0700, Jason Low wrote:
> >  __visible __used noinline
> > @@ -730,6 +744,23 @@ __mutex_unlock_common_slowpath(struct mutex *lock, int nested)
> >  	if (__mutex_slowpath_needs_to_unlock())
> >  		atomic_set(&lock->count, 1);
> >  
> > +/*
> > + * Skipping the mutex_has_owner() check when DEBUG, allows us to
> > + * avoid taking the wait_lock in order to do not call mutex_release()
> > + * and debug_mutex_unlock() when !DEBUG. This can otherwise result in
> > + * deadlocks when another task enters the lock's slowpath in mutex_lock().
> > + */
> > +#ifndef CONFIG_DEBUG_MUTEXES
> > +	/*
> > +	 * Abort the wakeup operation if there is an another mutex owner, as the
> > +	 * lock was stolen. mutex_unlock() should have cleared the owner field
> > +	 * before calling this function. If that field is now set, another task
> > +	 * must have acquired the mutex.
> > +	 */
> > +	if (mutex_has_owner(lock))
> > +		return;
> 
> Would we need the mutex lock count to eventually get set to a negative
> value if there are waiters? An optimistic spinner can get the lock and
> set lock->count to 0. Then the lock count might remain 0 since a waiter
> might not get waken up here to try-lock and set lock->count to -1 if it
> goes back to sleep in the lock path.

This is a good point, but I think we are safe because we do not rely on
strict dependence between the mutex counter and the wait list. So to see
if there are waiters to wakeup, we do a !list_empty() check, but to
determine the lock state, we rely on the counter.

Thanks,
Davidlohr

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