From: Linus Torvalds <torvalds@linux-foundation.org>
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: akpm@linux-foundation.org, Ingo Molnar <mingo@elte.hu>,
linux-kernel@vger.kernel.org,
KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
Steven Rostedt <rostedt@goodmis.org>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Nicholas Miell <nmiell@comcast.net>,
laijs@cn.fujitsu.com, dipankar@in.ibm.com, josh@joshtriplett.org,
dvhltc@us.ibm.com, niv@us.ibm.com, tglx@linutronix.de,
peterz@infradead.org, Valdis.Kletnieks@vt.edu,
dhowells@redhat.com
Subject: Re: [patch 2/3] scheduler: add full memory barriers upon task switch at runqueue lock/unlock
Date: Mon, 1 Feb 2010 07:27:16 -0800 (PST) [thread overview]
Message-ID: <alpine.LFD.2.00.1002010722350.4206@localhost.localdomain> (raw)
In-Reply-To: <20100131210013.446503342@polymtl.ca>
On Sun, 31 Jan 2010, Mathieu Desnoyers wrote:
>
> Adds no overhead on x86, because LOCK-prefixed atomic operations of the spin
> lock/unlock already imply a full memory barrier.
.. and as Nick pointed out, you're fundamentally incorrect on this.
unlock on x86 is no memory barrier at all, since the x86 memory ordering
rules are such that a regular store always has release consistency.
But more importantly, you don't even explain why the addded smp_mb()
helps.
Why does a smp_mb() at the lock/unlock even matter? Reading accesses by
the same CPU sure as hell do _not_ matter, so the whole concept seems
totally broken. There is no way in _hell_ that whatever unlocked thing
can ever write the variables protected by the lock, only read them. So a
full memory barrier makes zero sense to begin with.
So what are these magical memory barriers all about?
Linus
next prev parent reply other threads:[~2010-02-01 15:28 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-01-31 20:52 [patch 0/3] introduce sys_membarrier(): process-wide memory barrier (v8) Mathieu Desnoyers
2010-01-31 20:52 ` [patch 1/3] Create spin lock/spin unlock with distinct memory barrier Mathieu Desnoyers
2010-02-01 7:25 ` Nick Piggin
2010-02-01 14:08 ` Mathieu Desnoyers
2010-02-01 7:28 ` Nick Piggin
2010-02-01 14:10 ` Mathieu Desnoyers
2010-02-01 15:22 ` Linus Torvalds
2010-02-01 15:41 ` Steven Rostedt
2010-01-31 20:52 ` [patch 2/3] scheduler: add full memory barriers upon task switch at runqueue lock/unlock Mathieu Desnoyers
2010-02-01 7:33 ` Nick Piggin
2010-02-01 9:42 ` Peter Zijlstra
2010-02-01 10:11 ` Nick Piggin
2010-02-01 10:36 ` Peter Zijlstra
2010-02-01 10:49 ` Nick Piggin
2010-02-01 14:47 ` Mathieu Desnoyers
2010-02-01 14:58 ` Nick Piggin
2010-02-01 15:23 ` Steven Rostedt
2010-02-01 15:44 ` Steven Rostedt
2010-02-01 16:00 ` Mike Galbraith
2010-02-01 15:27 ` Linus Torvalds [this message]
2010-02-01 16:09 ` Mathieu Desnoyers
2010-02-01 16:23 ` Linus Torvalds
2010-02-01 16:48 ` Mathieu Desnoyers
2010-02-01 16:56 ` Linus Torvalds
2010-02-01 17:45 ` Mathieu Desnoyers
2010-02-01 18:00 ` Steven Rostedt
2010-02-01 18:36 ` Linus Torvalds
2010-02-01 19:56 ` Mathieu Desnoyers
2010-02-01 20:42 ` Linus Torvalds
2010-02-01 22:42 ` Mathieu Desnoyers
2010-02-01 20:33 ` Steven Rostedt
2010-02-01 20:52 ` Linus Torvalds
2010-02-01 22:39 ` Steven Rostedt
2010-02-01 23:09 ` Steven Rostedt
2010-02-01 17:13 ` Steven Rostedt
2010-02-01 17:34 ` Linus Torvalds
2010-02-01 16:24 ` Steven Rostedt
2010-02-01 16:29 ` Peter Zijlstra
2010-02-01 16:46 ` Steven Rostedt
2010-02-01 16:11 ` Steven Rostedt
2010-01-31 20:52 ` [patch 3/3] introduce sys_membarrier(): process-wide memory barrier (v8) Mathieu Desnoyers
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