From: Josh Triplett <josh@joshtriplett.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
linux-kernel@vger.kernel.org, mingo@kernel.org,
laijs@cn.fujitsu.com, dipankar@in.ibm.com,
akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
niv@us.ibm.com, tglx@linutronix.de, rostedt@goodmis.org,
dhowells@redhat.com, edumazet@google.com, darren@dvhart.com,
fweisbec@gmail.com, sbw@mit.edu, Ingo Molnar <mingo@redhat.com>,
Oleg Nesterov <oleg@redhat.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Will Deacon <will.deacon@arm.com>,
Tim Chen <tim.c.chen@linux.intel.com>,
Waiman Long <waiman.long@hp.com>,
Andrea Arcangeli <aarcange@redhat.com>,
Andi Kleen <andi@firstfloor.org>,
Michel Lespinasse <walken@google.com>,
Davidlohr Bueso <davidlohr.bueso@hp.com>,
Rik van Riel <riel@redhat.com>,
Peter Hurley <peter@hurleysoftware.com>,
"H. Peter Anvin" <hpa@zytor.com>, Arnd Bergmann <arnd@arndb.de>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: Re: [PATCH v5 tip/core/locking 5/7] Documentation/memory-barriers.txt: Downgrade UNLOCK+LOCK
Date: Tue, 10 Dec 2013 09:43:20 -0800 [thread overview]
Message-ID: <20131210174320.GB10311@leaf> (raw)
In-Reply-To: <20131210172528.GQ12849@twins.programming.kicks-ass.net>
On Tue, Dec 10, 2013 at 06:25:28PM +0100, Peter Zijlstra wrote:
> On Tue, Dec 10, 2013 at 09:12:47AM -0800, Paul E. McKenney wrote:
> > > The way I read the above it says that you need
> > > smp_mb__after_unlock_lock() when the UNLOCK and LOCK are on the same
> > > variable. That doesn't make sense, I thought that was the one case we
> > > all agreed on it would indeed be a full barrier without extra trickery.
> >
> > On x86, sure, but smp_mb__after_unlock_lock() is nothingness on x86
> > anyway. Other architectures might benefit from requiring that the
> > smp_mb__after_unlock_lock() be used in this case.
>
> Confused, UNLOCK X, LOCK X, must always be fully serializing. That's the
> entire purpose of the thing.
>
> The only place you can go play games (and clearly we are going there) is
> when the UNLOCK and LOCK are on different variables.
That would certainly be a good assumption to preserve, and it would
eliminate most of the need for smp_mb__after_unlock_lock().
- Josh Triplett
next prev parent reply other threads:[~2013-12-10 17:43 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-12-10 1:27 [PATCH v5 tip/core/locking] Memory-barrier documentation updates + smp_mb__after_unlock_lock() Paul E. McKenney
2013-12-10 1:27 ` [PATCH v5 tip/core/locking 1/7] Documentation/memory-barriers.txt: Add needed ACCESS_ONCE() calls to memory-barriers.txt Paul E. McKenney
2013-12-10 1:27 ` [PATCH v5 tip/core/locking 2/7] Documentation/memory-barriers.txt: Add long atomic examples " Paul E. McKenney
2013-12-10 1:27 ` [PATCH v5 tip/core/locking 3/7] Documentation/memory-barriers.txt: Prohibit speculative writes Paul E. McKenney
2013-12-10 1:28 ` [PATCH v5 tip/core/locking 4/7] Documentation/memory-barriers.txt: Document ACCESS_ONCE() Paul E. McKenney
2013-12-10 1:28 ` [PATCH v5 tip/core/locking 5/7] Documentation/memory-barriers.txt: Downgrade UNLOCK+LOCK Paul E. McKenney
2013-12-10 1:32 ` Josh Triplett
2013-12-10 5:19 ` Paul E. McKenney
2013-12-10 13:14 ` Peter Zijlstra
2013-12-10 17:12 ` Paul E. McKenney
2013-12-10 17:25 ` Peter Zijlstra
2013-12-10 17:43 ` Josh Triplett [this message]
2013-12-10 18:05 ` Paul E. McKenney
2013-12-10 17:49 ` Paul E. McKenney
2013-12-10 17:43 ` Peter Zijlstra
2013-12-10 18:49 ` Paul E. McKenney
2013-12-10 16:44 ` Oleg Nesterov
2013-12-10 17:15 ` Paul E. McKenney
2013-12-10 17:35 ` Oleg Nesterov
2013-12-10 1:28 ` [PATCH v5 tip/core/locking 6/7] locking: Add an smp_mb__after_unlock_lock() for UNLOCK+LOCK barrier Paul E. McKenney
2013-12-10 1:34 ` Josh Triplett
2013-12-10 5:26 ` Paul E. McKenney
2013-12-10 18:53 ` Paul E. McKenney
2013-12-10 12:37 ` Peter Zijlstra
2013-12-10 17:17 ` Paul E. McKenney
2013-12-10 17:45 ` Josh Triplett
2013-12-10 20:11 ` Paul E. McKenney
2013-12-10 17:04 ` Oleg Nesterov
2013-12-10 17:18 ` Paul E. McKenney
2013-12-10 17:32 ` Oleg Nesterov
2013-12-10 1:28 ` [PATCH v5 tip/core/locking 7/7] rcu: Apply smp_mb__after_unlock_lock() to preserve grace periods Paul E. McKenney
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