linux-kernel - Re: [PATCH 3/3] MIPS: bugfix - replace smp_mb with release barrier function in unlocks

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Date:	Tue, 2 Jun 2015 15:22:43 +0200
From:	Ralf Baechle <ralf@...ux-mips.org>
To:	James Hogan <james.hogan@...tec.com>
Cc:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>,
	linux-mips@...ux-mips.org, benh@...nel.crashing.org,
	will.deacon@....com, linux-kernel@...r.kernel.org,
	markos.chandras@...tec.com, macro@...ux-mips.org,
	Steven.Hill@...tec.com, alexander.h.duyck@...hat.com,
	davem@...emloft.net
Subject: Re: [PATCH 3/3] MIPS: bugfix - replace smp_mb with release barrier
 function in unlocks

On Tue, Jun 02, 2015 at 12:42:40PM +0100, James Hogan wrote:

> Replace.
> 
> > smp_mb__before_llsc() call which does "release" barrier functionality.
> > 
> > It seems like it was missed in commit f252ffd50c97dae87b45f1dbad24f71358ccfbd6
> > during introduction of "acquire" and "release" semantics.
> > 
> > Signed-off-by: Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
> > ---
> >  arch/mips/include/asm/bitops.h   |    2 +-
> >  arch/mips/include/asm/spinlock.h |    2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/mips/include/asm/bitops.h b/arch/mips/include/asm/bitops.h
> > index 0cf29bd5dc5c..ce9666cf1499 100644
> > --- a/arch/mips/include/asm/bitops.h
> > +++ b/arch/mips/include/asm/bitops.h
> > @@ -469,7 +469,7 @@ static inline int test_and_change_bit(unsigned long nr,
> >   */
> >  static inline void __clear_bit_unlock(unsigned long nr, volatile unsigned long *addr)
> >  {
> > -	smp_mb();
> > +	smp_mb__before_llsc();
> >  	__clear_bit(nr, addr);
> >  }
> >  
> > diff --git a/arch/mips/include/asm/spinlock.h b/arch/mips/include/asm/spinlock.h
> > index 1fca2e0793dc..7c7f3b2bd3de 100644
> > --- a/arch/mips/include/asm/spinlock.h
> > +++ b/arch/mips/include/asm/spinlock.h
> > @@ -317,7 +317,7 @@ static inline void arch_write_lock(arch_rwlock_t *rw)
> >  
> >  static inline void arch_write_unlock(arch_rwlock_t *rw)
> >  {
> > -	smp_mb();
> > +	smp_mb__before_llsc();
> 
> arch_write_unlock appears to just use sw, not sc, and __clear_bit
> appears to be implemented in plain C, so is smp_mb__before_llsc() really
> appropriate? Would smp_release() be more accurate/correct in both cases?

Yes on the both questions.

  Ralf
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