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

[Jason Low <jason.low2@hp.com>]

On Fri, 2014-08-08 at 13:21 -0700, Davidlohr Bueso wrote:
> 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.

Right, though if an optimistic spinner gets the lock, it would set
lock->count to 0. After it is done with its critical region and calls
mutex_unlock(), it would skip the slowpath and not wake up the next
thread either, because it sees that the lock->count is 0. In that case,
there might be a situation where the following mutex_unlock() call would
skip waking up the waiter as there's no call to slowpath.