Re: atomic_ops(3) and sync

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Wed, Jun 16, 2010 at 12:21:23PM +0200, Manuel Bouyer wrote:
> > This behaviour is documented in atomic_ops(3) (your email does not
> > provide clear indication as to if you read that part of the manpage,
> > so I'll just quote it here):
> > 
> >     Atomic operations are strongly ordered with respect to each other.
> >     The global visibility of other loads and stores before and after an
> >     atomic operation is undefined.  Applications that require synchro-
> >     nization of loads and stores with respect to an atomic operation must
> >     use memory barriers.  See membar_ops(3).
> > 
> > So if you want to impose ordering on other accesses with respect to the
> > atomic op you need to manually add membars.  Atomic itself does not,
> > and should not, guarantee it.
> 
> OK, thanks. I didn't spot this from the man page. I'm still in
> a x86-centric world I guess ...

But it seems there's a problem with our membar_ops too:
(gdb) disas membar_enter
Dump of assembler code for function membar_sync:
0xffffffffc02b1b70 <membar_sync+0>:     jr      ra
0xffffffffc02b1b74 <membar_sync+4>:     nop
0xffffffffc02b1b78 <membar_sync+8>:     sync
0xffffffffc02b1b7c <membar_sync+12>:    nop

the assembler inserted a nop in the delay slot; I guess membar_ops.S
needs ".set noreorder" at the top.

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--