linux-kernel - Re: allow preemption in check_task

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Message-ID: <20140210173833.GQ9987@twins.programming.kicks-ass.net>
Date:	Mon, 10 Feb 2014 18:38:33 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Nicholas Mc Guire <der.herr@...r.at>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	linux-rt-users@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Carsten Emde <C.Emde@...dl.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andreas Platschek <platschek@....tuwien.ac.at>
Subject: Re: allow preemption in check_task_state

On Mon, Feb 10, 2014 at 06:17:12PM +0100, Nicholas Mc Guire wrote:
> maybe I'm missing/missunderstanding something here but
> pi_unlock -> arch_spin_unlock is a full mb() 

Nope, arch_spin_unlock() on x86 is a single add[wb] without LOCK prefix.

The lock and unlock primitives are in general specified to have ACQUIRE
resp. RELEASE semantics.

See Documentation/memory-barriers.txt for far too much head-hurting
details.
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