linux-kernel - Re: [PATCHv2 2/2] memory barrier: adding smp_mb__after

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Message-ID: <20090630121318.GA4406@jolsa.lab.eng.brq.redhat.com>
Date:	Tue, 30 Jun 2009 14:13:18 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	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: [PATCHv2 2/2] memory barrier: adding smp_mb__after_lock

On Tue, Jun 30, 2009 at 01:14:20PM +0200, Eric Dumazet wrote:
> Jiri Olsa a écrit :
> > Adding smp_mb__after_lock define to be used as a smp_mb call after
> > a lock.  
> > 
> > Making it nop for x86, since {read|write|spin}_lock() on x86 are 
> > full memory barriers.
> > 
> > wbr,
> > jirka
> > 
> > 
> > Signed-off-by: Jiri Olsa <jolsa@...hat.com>
> > 
> > ---
> >  arch/x86/include/asm/spinlock.h |    3 +++
> >  include/linux/spinlock.h        |    5 +++++
> >  include/net/sock.h              |    2 +-
> >  3 files changed, 9 insertions(+), 1 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
> > index b7e5db8..39ecc5f 100644
> > --- a/arch/x86/include/asm/spinlock.h
> > +++ 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)
> > +
> >  #endif /* _ASM_X86_SPINLOCK_H */
> > diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
> > index 252b245..ae053bd 100644
> > --- a/include/linux/spinlock.h
> > +++ b/include/linux/spinlock.h
> > @@ -132,6 +132,11 @@ do {								\
> >  #endif /*__raw_spin_is_contended*/
> >  #endif
> >  
> > +/* The lock does not imply full memory barrier. */
> > +#ifndef smp_mb__after_lock
> > +#define smp_mb__after_lock() smp_mb()
> > +#endif
> > +
> >  /**
> >   * spin_unlock_wait - wait until the spinlock gets unlocked
> >   * @lock: the spinlock in question.
> > diff --git a/include/net/sock.h b/include/net/sock.h
> > index a12df10..0d57e83 100644
> > --- a/include/net/sock.h
> > +++ b/include/net/sock.h
> > @@ -1277,7 +1277,7 @@ static inline void sock_poll_wait(struct file *filp,
> >  		 *
> >  		 * This memory barrier is paired in the sk_has_sleeper.
> >  		*/
> > -		smp_mb();
> > +		smp_mb__after_lock();
> >  	}
> >  }
> 
> I believe you took wrong point to use this new thing :)
> 
> It was meant to be used in sk_has_sleeper() only (as sk_has_sleeper()
>  follows a read_lock())
>

shoot, you're right.. I'll resend 2/2, thanks

jirka

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