Le lundi 06 juin 2011 Ã 14:23 -0700, Andrew Morton a Ãcrit :
> On Sun, 29 May 2011 23:19:28 -0400
> Mike Frysinger <vapier@xxxxxxxxxx> wrote:
>
> > To make these guys work on SMP systems, we just need to sprinkle a few
> > barriers around.
> >
> > Signed-off-by: Mike Frysinger <vapier@xxxxxxxxxx>
> > ---
> > 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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