From: Peter Zijlstra <peterz@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Davidlohr Bueso <davidlohr@hp.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@redhat.com>,
Darren Hart <dvhart@linux.intel.com>,
Thomas Gleixner <tglx@linutronix.de>,
Paul McKenney <paulmck@linux.vnet.ibm.com>,
Mike Galbraith <efault@gmx.de>, Jeff Mahoney <jeffm@suse.com>,
Jason Low <jason.low2@hp.com>, Waiman Long <Waiman.Long@hp.com>,
Tom Vaden <tom.vaden@hp.com>,
"Norton, Scott J" <scott.norton@hp.com>,
"Chandramouleeswaran, Aswin" <aswin@hp.com>,
Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH v3 4/4] futex: Avoid taking hb lock if nothing to wakeup
Date: Fri, 20 Dec 2013 09:54:26 +0100 [thread overview]
Message-ID: <20131220085426.GZ21999@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <CA+55aFyrZmG6rNpY22OK3UQi18e=4UQyh8a3tpxY+QVCbNxXkw@mail.gmail.com>
On Thu, Dec 19, 2013 at 07:13:09PM -0800, Linus Torvalds wrote:
> I think we might have to order the two reads with an smp_rmb - making
> sure that we check the lock state first - but I think it should be
> otherwise pretty solid.
> Yeah, I said "spin_contended()" myself initially, but it needs to be
> "spin_is_locked()". It's hopefully unlikely to ever actually be
> contended (which in ticket lock terms is "head is _more_ than one away
> from the tail").
Right, which would give us something like:
LOCK m is_locked m
list_{add,del} RMB
UNLOCK m list_empty
Where the rmb matches the release/unlock. This guarantees that when we
see the unlock we must also see the list modification.
next prev parent reply other threads:[~2013-12-20 8:54 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-12-19 18:45 [PATCH v3 0/4] futex: Wakeup optimizations Davidlohr Bueso
2013-12-19 18:45 ` [PATCH v3 1/4] futex: Misc cleanups Davidlohr Bueso
2013-12-19 18:45 ` [PATCH v3 2/4] futex: Larger hash table Davidlohr Bueso
2013-12-19 18:45 ` [PATCH v3 3/4] futex: Document ordering guarantees Davidlohr Bueso
2013-12-19 18:45 ` [PATCH v3 4/4] futex: Avoid taking hb lock if nothing to wakeup Davidlohr Bueso
2013-12-19 19:25 ` Ingo Molnar
2013-12-19 19:29 ` Davidlohr Bueso
2013-12-19 19:42 ` Ingo Molnar
2013-12-19 22:35 ` Joe Perches
2013-12-20 9:11 ` Peter Zijlstra
2013-12-20 10:27 ` Ingo Molnar
2013-12-19 23:14 ` Linus Torvalds
2013-12-19 23:42 ` Davidlohr Bueso
2013-12-19 23:53 ` Linus Torvalds
2013-12-20 0:04 ` Linus Torvalds
2013-12-20 0:26 ` Darren Hart
2013-12-20 0:12 ` Linus Torvalds
2013-12-20 2:22 ` Davidlohr Bueso
2013-12-20 3:13 ` Linus Torvalds
2013-12-20 6:13 ` Davidlohr Bueso
2013-12-20 8:54 ` Peter Zijlstra [this message]
2013-12-20 19:30 ` Davidlohr Bueso
2013-12-20 19:54 ` Linus Torvalds
2013-12-21 5:57 ` Davidlohr Bueso
2013-12-21 1:36 ` Davidlohr Bueso
2013-12-21 2:34 ` Darren Hart
2013-12-21 3:08 ` Davidlohr Bueso
2013-12-25 3:13 ` Waiman Long
2014-01-02 11:37 ` Davidlohr Bueso
-- strict thread matches above, loose matches on Subject: below --
2013-12-03 9:45 [PATCH v2 0/4] futex: Wakeup optimizations Davidlohr Bueso
2013-12-03 9:45 ` [PATCH v2 4/4] futex: Avoid taking hb lock if nothing to wakeup Davidlohr Bueso
2013-12-10 18:10 ` [PATCH v3 " Davidlohr Bueso
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