Messages in this thread | | | Subject | Re: [PATCH/RFC] asm-generic/mutex-dec.h: add SMP support | From | Eric Dumazet <> | Date | Mon, 06 Jun 2011 23:32:58 +0200 |
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Le lundi 06 juin 2011 à 14:23 -0700, Andrew Morton a écrit : > On Sun, 29 May 2011 23:19:28 -0400 > Mike Frysinger <vapier@gentoo.org> wrote: > > > To make these guys work on SMP systems, we just need to sprinkle a few > > barriers around. > > > > Signed-off-by: Mike Frysinger <vapier@gentoo.org> > > --- > > note: this is what the Blackfin SMP port is using, but it doesn't seem > > like other SMP ports are ... so I wonder if we're just trying too hard > > and these barriers aren't actually necessary ? > > > > include/asm-generic/mutex-dec.h | 8 +++++++- > > 1 files changed, 7 insertions(+), 1 deletions(-) > > > > diff --git a/include/asm-generic/mutex-dec.h b/include/asm-generic/mutex-dec.h > > index f104af7..e746c3c 100644 > > --- a/include/asm-generic/mutex-dec.h > > +++ b/include/asm-generic/mutex-dec.h > > @@ -22,6 +22,8 @@ __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *)) > > { > > if (unlikely(atomic_dec_return(count) < 0)) > > fail_fn(count);
Check Documentation/memory-barriers.txt, around line 1688
atomic_dec_return() implies a full memory barrier on each side of the operation.
smp_mb() is therefore not needed here
> > + else > > + smp_mb(); > > } > > > > /** > > @@ -39,6 +41,7 @@ __mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *)) > > { > > if (unlikely(atomic_dec_return(count) < 0)) > > return fail_fn(count); > > + smp_mb();
atomic_dec_return() implies a full memory barrier. smp_mb() is therefore not needed here
> > return 0; > > } > > > > @@ -58,6 +61,7 @@ __mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *)) > > static inline void > > __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *)) > > { > > + smp_mb();
atomic_inc_return() implies a full memory barrier. smp_mb() is therefore not needed here
> > if (unlikely(atomic_inc_return(count) <= 0)) > > fail_fn(count); > > } > > @@ -82,8 +86,10 @@ __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *)) > > static inline int > > __mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *)) > > { > > - if (likely(atomic_cmpxchg(count, 1, 0) == 1)) > > + if (likely(atomic_cmpxchg(count, 1, 0) == 1)) { > > + smp_mb();
atomic_cmpxchg() implies a full memory barrier. smp_mb() is therefore not needed here
> > return 1; > > + } > > return 0; > > } > > This patch basically reverts Nick's a8ddac7e53e89cb ("mutex: speed up > generic mutex implementations"). What's up with that? > > I could try to review this patch but I'm pathetic with barriers. Help.
Well, I really dont understand this patch, it makes no sense.
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