linux-kernel - Re: [patch 1/3] Create spin lock/spin unlock with distinct memory barrier

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Message-ID: <20100201141010.GB10894@Krystal>
Date:	Mon, 1 Feb 2010 09:10:10 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Nicholas Miell <nmiell@...cast.net>, laijs@...fujitsu.com,
	dipankar@...ibm.com, josh@...htriplett.org, dvhltc@...ibm.com,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com
Subject: Re: [patch 1/3] Create spin lock/spin unlock with distinct memory
	barrier

* Nick Piggin (npiggin@...e.de) wrote:
> On Sun, Jan 31, 2010 at 03:52:55PM -0500, Mathieu Desnoyers wrote:
> > +/*
> > + * X86 spinlock-mb mappings. Use standard spinlocks with acquire/release
> > + * semantics. Associated memory barriers are defined as no-ops, because the
> > + * spinlock LOCK-prefixed atomic operations imply a full memory barrier.
> > + */
> > +
> > +#define spin_lock__no_acquire			spin_lock
> > +#define spin_unlock__no_release			spin_unlock
> > +
> > +#define spin_lock_irq__no_acquire		spin_lock_irq
> > +#define spin_unlock_irq__no_release		spin_unlock_irq
> > +
> > +#define raw_spin_lock__no_acquire		raw_spin_lock
> > +#define raw_spin_unlock__no_release		raw_spin_unlock
> > +
> > +#define raw_spin_lock_irq__no_acquire		raw_spin_lock_irq
> > +#define raw_spin_unlock_irq__no_release		raw_spin_unlock_irq
> > +
> > +#define smp_acquire__after_spin_lock()		do { } while (0)
> > +#define smp_release__before_spin_unlock()	do { } while (0)
> > +
> > +#define smp_mb__after_spin_lock()		do { } while (0)
> > +#define smp_mb__before_spin_unlock()		do { } while (0)
> 
> Oh, and that one's wrong. loads can pass spin_unlock on x86 so it
> needs to be smp_mb().
> 

Good catch !

#if defined(CONFIG_X86_32) && \
        (defined(CONFIG_X86_OOSTORE) || defined(CONFIG_X86_PPRO_FENCE))
/*
 * On PPro SMP or if we are using OOSTORE, we use a locked operation to unlock
 * (PPro errata 66, 92)
 */
# define UNLOCK_LOCK_PREFIX LOCK_PREFIX
#else
# define UNLOCK_LOCK_PREFIX
#endif

Thanks,

Mathieu


-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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