From: Davidlohr Bueso <davidlohr@hp.com>
To: paulmck@linux.vnet.ibm.com
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org,
dvhart@linux.intel.com, peterz@infradead.org, tglx@linutronix.de,
efault@gmx.de, jeffm@suse.com, torvalds@linux-foundation.org,
jason.low2@hp.com, Waiman.Long@hp.com, tom.vaden@hp.com,
scott.norton@hp.com, aswin@hp.com
Subject: Re: [PATCH v5 4/4] futex: Avoid taking hb lock if nothing to wakeup
Date: Sat, 11 Jan 2014 10:21:44 -0800 [thread overview]
Message-ID: <1389464504.21727.44.camel@buesod1.americas.hpqcorp.net> (raw)
In-Reply-To: <20140111095236.GA1181@linux.vnet.ibm.com>
On Sat, 2014-01-11 at 01:52 -0800, Paul E. McKenney wrote:
[...]
> On Sat, Jan 11, 2014 at 01:49:12AM -0800, Paul E. McKenney wrote:
> > On Thu, Jan 02, 2014 at 07:05:20AM -0800, Davidlohr Bueso wrote:
> > > - spin_lock(&hb->lock);
> > > + spin_lock(&hb->lock); /* implies MB (A) */
> >
> > You need smp_mb__before_spinlock() before the spin_lock() to get a
> > full memory barrier.
Hmmm, the thing we need to guarantee here is that the ticket increment
is visible (which is the same as the smp_mb__after_atomic_inc we used to
have in the original atomic counter approach), so adding a barrier
before the spin_lock call wouldn't serve that. I previously consulted
this with Linus and we can rely on the fact that spin_lock calls already
update the head counter, so spinners are visible even if the lock hasn't
been acquired yet.
> Actually, even that only gets you smp_mb().
I guess you mean smp_wmb() here.
> Unless you are ordering a prior write against a later write here, you
> will need an smp_mb().
Yep.
Thanks for looking into this,
Davidlohr
next prev parent reply other threads:[~2014-01-11 18:21 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-01-02 15:05 [PATCH v5 0/4] futex: Wakeup optimizations Davidlohr Bueso
2014-01-02 15:05 ` [PATCH v5 1/4] futex: Misc cleanups Davidlohr Bueso
2014-01-11 6:43 ` Paul E. McKenney
2014-01-02 15:05 ` [PATCH v5 2/4] futex: Larger hash table Davidlohr Bueso
2014-01-11 7:37 ` Paul E. McKenney
2014-01-02 15:05 ` [PATCH v5 3/4] futex: Document ordering guarantees Davidlohr Bueso
2014-01-06 18:58 ` Darren Hart
2014-01-11 7:40 ` Paul E. McKenney
2014-01-02 15:05 ` [PATCH v5 4/4] futex: Avoid taking hb lock if nothing to wakeup Davidlohr Bueso
2014-01-02 19:23 ` Linus Torvalds
2014-01-02 20:59 ` Davidlohr Bueso
2014-01-06 20:56 ` Darren Hart
2014-01-06 20:52 ` Darren Hart
2014-01-07 3:29 ` Davidlohr Bueso
2014-01-07 17:40 ` Darren Hart
2014-01-11 9:49 ` Paul E. McKenney
2014-01-11 9:52 ` Paul E. McKenney
2014-01-11 18:21 ` Davidlohr Bueso [this message]
2014-01-06 0:59 ` [PATCH v5 0/4] futex: Wakeup optimizations Davidlohr Bueso
2014-01-06 1:38 ` [PATCH 5/4] futex: silence uninitialized warnings Davidlohr Bueso
2014-01-06 18:48 ` Darren Hart
2014-01-07 2:55 ` Linus Torvalds
2014-01-07 3:02 ` Davidlohr Bueso
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