Re: [PATCH v5 4/4] qrwlock: Use the mcs_spinlock helper functionsfor MCS queuing

[Waiman Long]

On 11/08/2013 04:21 PM, Paul E. McKenney wrote:
> On Mon, Nov 04, 2013 at 12:17:20PM -0500, Waiman Long wrote:
> > There is a pending patch in the rwsem patch series that adds a generic
> > MCS locking helper functions to do MCS-style locking. This patch
> > will enable the queue rwlock to use that generic MCS lock/unlock
> > primitives for internal queuing. This patch should only be merged
> > after the merging of that generic MCS locking patch.
> >
> > Signed-off-by: Waiman Long<Waiman.Long@xxxxxx>
> This one does might address at least some of the earlier memory-barrier
> issues, at least assuming that the MCS lock is properly memory-barriered.
>
> Then again, maybe not.  Please see below.
>
> 							Thanx, Paul
>
> >   	/*
> >   	 * At the head of the wait queue now, try to increment the reader
> > @@ -172,12 +103,36 @@ void queue_read_lock_slowpath(struct qrwlock *lock)
> >   		while (ACCESS_ONCE(lock->cnts.writer))
> >   			cpu_relax();
> >   	}
> > -	rspin_until_writer_unlock(lock, 1);
> > -	signal_next(lock,&node);
> > +	/*
> > +	 * Increment reader count&  wait until writer unlock
> > +	 */
> > +	cnts.rw = xadd(&lock->cnts.rw, QRW_READER_BIAS);
> > +	rspin_until_writer_unlock(lock, cnts);
> > +	mcs_spin_unlock(&lock->waitq,&node);
> But mcs_spin_unlock() is only required to do a RELEASE barrier, which
> could still allow critical-section leakage.

Yes, that is a problem. I will try to add an ACQUIRE barrier in reading 
the writer byte.

-Longman

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