Messages in this thread | | | Date | Tue, 24 Jul 2007 13:53:14 +1000 | From | Nick Piggin <> | Subject | Re: [PATCH 8/8] i386: bitops: smp_mb__{before, after}_clear_bit() definitions |
| |
Satyam Sharma wrote: > From: Satyam Sharma <ssatyam@cse.iitk.ac.in> > > [8/8] i386: bitops: smp_mb__{before, after}_clear_bit() definitions > >>From Documentation/atomic_ops.txt, those archs that require explicit > memory barriers around clear_bit() must also implement these two interfaces. > However, for i386, clear_bit() is a strict, locked, atomic and > un-reorderable operation and includes an implicit memory barrier already. > > But these two functions have been wrongly defined as "barrier()" which is > a pointless _compiler optimization_ barrier, and only serves to make gcc > not do legitimate optimizations that it could have otherwise done. > > So let's make these proper no-ops, because that's exactly what we require > these to be on the i386 platform.
No. clear_bit is not a compiler barrier on i386, thus smp_mb__before/after must be.
> > Signed-off-by: Satyam Sharma <ssatyam@cse.iitk.ac.in> > Cc: David Howells <dhowells@redhat.com> > Cc: Nick Piggin <nickpiggin@yahoo.com.au> > > --- > > [ A similar optimization needs to be done in the atomic.h also. > Will submit that patch shortly. ] > > include/asm-i386/bitops.h | 4 ++-- > 1 files changed, 2 insertions(+), 2 deletions(-) > > diff --git a/include/asm-i386/bitops.h b/include/asm-i386/bitops.h > index 4f1fda5..42999eb 100644 > --- a/include/asm-i386/bitops.h > +++ b/include/asm-i386/bitops.h > @@ -106,8 +106,8 @@ static inline void __clear_bit(int nr, unsigned long *addr) > * Bit operations are already serializing on x86. > * These must still be defined here for API completeness. > */ > -#define smp_mb__before_clear_bit() barrier() > -#define smp_mb__after_clear_bit() barrier() > +#define smp_mb__before_clear_bit() do {} while (0) > +#define smp_mb__after_clear_bit() do {} while (0) > > /** > * __change_bit - Toggle a bit in memory >
-- SUSE Labs, Novell Inc. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |