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Date: Mon, 10 Feb 2014 19:16:08 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Nicholas Mc Guire <der.herr@hofr.at>
Cc: Steven Rostedt <rostedt@goodmis.org>, linux-rt-users@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Carsten Emde <C.Emde@osadl.org>, Thomas Gleixner <tglx@linutronix.de>,
        Andreas Platschek <platschek@ict.tuwien.ac.at>
Subject: Re: allow preemption in check_task_state
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On Mon, Feb 10, 2014 at 07:12:03PM +0100, Nicholas Mc Guire wrote:
> On Mon, 10 Feb 2014, Peter Zijlstra wrote:
> 
> > 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.
> 
> I did check that - but from the code check it seems to me to be using a
> lock prefix in the fast __add() path and an explicit smp_add() in the slow
> path (arch/x86/include/asm/spinlock.h:arch_spin_unlock) the __add from 
> arch/x86/include/asm/cmpxchg.h does lock or am I missinterpreting this ?
> the other archs I believe were all doing explicit mb()/smp_mb() in the 
> arch_spin_unlock - will go check this again.

It uses UNLOCK_LOCK_PREFIX, which if you look carefully, is normally
always "". Only some 'broken' i386 chips require a LOCK there.