netdev - Re: [PATCHv5 2/2] memory barrier: adding smp_mb__after

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Date:	Fri, 3 Jul 2009 12:25:30 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	fbl@...hat.com, nhorman@...hat.com, davem@...hat.com,
	htejun@...il.com, jarkao2@...il.com, oleg@...hat.com,
	davidel@...ilserver.org
Subject: Re: [PATCHv5 2/2] memory barrier: adding smp_mb__after_lock


* Jiri Olsa <jolsa@...hat.com> wrote:

> On Fri, Jul 03, 2009 at 11:24:38AM +0200, Ingo Molnar wrote:
> > 
> > * Eric Dumazet <eric.dumazet@...il.com> wrote:
> > 
> > > Ingo Molnar a écrit :
> > > > * Jiri Olsa <jolsa@...hat.com> wrote:
> > > > 
> > > >> +++ b/arch/x86/include/asm/spinlock.h
> > > >> @@ -302,4 +302,7 @@ static inline void __raw_write_unlock(raw_rwlock_t *rw)
> > > >>  #define _raw_read_relax(lock)	cpu_relax()
> > > >>  #define _raw_write_relax(lock)	cpu_relax()
> > > >>  
> > > >> +/* The {read|write|spin}_lock() on x86 are full memory barriers. */
> > > >> +#define smp_mb__after_lock() do { } while (0)
> > > > 
> > > > Two small stylistic comments, please make this an inline function:
> > > > 
> > > > static inline void smp_mb__after_lock(void) { }
> > > > #define smp_mb__after_lock
> > > > 
> > > > (untested)
> > > > 
> > > >> +/* The lock does not imply full memory barrier. */
> > > >> +#ifndef smp_mb__after_lock
> > > >> +#define smp_mb__after_lock() smp_mb()
> > > >> +#endif
> > > > 
> > > > ditto.
> > > > 
> > > > 	Ingo
> > > 
> > > This was following existing implementations of various smp_mb__??? helpers :
> > > 
> > > # grep -4 smp_mb__before_clear_bit include/asm-generic/bitops.h
> > > 
> > > /*
> > >  * clear_bit may not imply a memory barrier
> > >  */
> > > #ifndef smp_mb__before_clear_bit
> > > #define smp_mb__before_clear_bit()      smp_mb()
> > > #define smp_mb__after_clear_bit()       smp_mb()
> > > #endif
> > 
> > Did i mention that those should be fixed too? :-)
> > 
> > 	Ingo
> 
> ok, could I include it in the 2/2 or you prefer separate patch?

depends on whether it will regress ;-)

If it regresses, it's better to have it separate. If it wont, it can 
be included. If unsure, default to the more conservative option.

	Ingo
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