> Date: Mon, 16 Oct 2017 11:53:30 +0200
> From: Martin Pieuchot <m...@openbsd.org>
>
> On 16/10/17(Mon) 11:36, Mark Kettenis wrote:
> > > Date: Mon, 16 Oct 2017 10:52:09 +0200
> > > From: Martin Pieuchot <m...@openbsd.org>
> > >
> > > As pointed by Mateusz Guzik [0], on x86 the cmpxchg{q,l} used by
> > > rw_enter(9) and rw_exit(9) already include an implicit memory
> > > barrier, so we can avoids using an explicit expensive one by
> > > using the following variants.
> > >
> > > [0] https://marc.info/?l=openbsd-tech&m=150765959923113&w=2
> > >
> > > ok?
> >
> > Is this really safe? The atomic instructions are executed
> > conditionally...
>
> I guess you're talking about the "if ()" dance in the "optimized"
> versions: rw_{enter,exit}_{read,write}(). If no cmpxchg{q,l} is
> executed in theses functions _rw_{enter,exit}() will be called,
> which contains another barrier and another CAS.
Then it should be fine.
> > > Index: kern/kern_rwlock.c
> > > ===================================================================
> > > RCS file: /cvs/src/sys/kern/kern_rwlock.c,v
> > > retrieving revision 1.31
> > > diff -u -p -r1.31 kern_rwlock.c
> > > --- kern/kern_rwlock.c 12 Oct 2017 09:19:45 -0000 1.31
> > > +++ kern/kern_rwlock.c 16 Oct 2017 08:24:27 -0000
> > > @@ -96,7 +96,7 @@ _rw_enter_read(struct rwlock *rwl LOCK_F
> > > rw_cas(&rwl->rwl_owner, owner, owner + RWLOCK_READ_INCR)))
> > > _rw_enter(rwl, RW_READ LOCK_FL_ARGS);
> > > else {
> > > - membar_enter();
> > > + membar_enter_after_atomic();
> > > WITNESS_CHECKORDER(&rwl->rwl_lock_obj, LOP_NEWORDER, file, line,
> > > NULL);
> > > WITNESS_LOCK(&rwl->rwl_lock_obj, 0, file, line);
> > > @@ -112,7 +112,7 @@ _rw_enter_write(struct rwlock *rwl LOCK_
> > > RW_PROC(p) | RWLOCK_WRLOCK)))
> > > _rw_enter(rwl, RW_WRITE LOCK_FL_ARGS);
> > > else {
> > > - membar_enter();
> > > + membar_enter_after_atomic();
> > > WITNESS_CHECKORDER(&rwl->rwl_lock_obj,
> > > LOP_EXCLUSIVE | LOP_NEWORDER, file, line, NULL);
> > > WITNESS_LOCK(&rwl->rwl_lock_obj, LOP_EXCLUSIVE, file, line);
> > > @@ -126,7 +126,7 @@ _rw_exit_read(struct rwlock *rwl LOCK_FL
> > >
> > > rw_assert_rdlock(rwl);
> > >
> > > - membar_exit();
> > > + membar_exit_before_atomic();
> > > if (__predict_false((owner & RWLOCK_WAIT) ||
> > > rw_cas(&rwl->rwl_owner, owner, owner - RWLOCK_READ_INCR)))
> > > _rw_exit(rwl LOCK_FL_ARGS);
> > > @@ -141,7 +141,7 @@ _rw_exit_write(struct rwlock *rwl LOCK_F
> > >
> > > rw_assert_wrlock(rwl);
> > >
> > > - membar_exit();
> > > + membar_exit_before_atomic();
> > > if (__predict_false((owner & RWLOCK_WAIT) ||
> > > rw_cas(&rwl->rwl_owner, owner, 0)))
> > > _rw_exit(rwl LOCK_FL_ARGS);
> > > @@ -261,7 +261,7 @@ retry:
> > >
> > > if (__predict_false(rw_cas(&rwl->rwl_owner, o, o + inc)))
> > > goto retry;
> > > - membar_enter();
> > > + membar_enter_after_atomic();
> > >
> > > /*
> > > * If old lock had RWLOCK_WAIT and RWLOCK_WRLOCK set, it means we
> > > @@ -295,7 +295,7 @@ _rw_exit(struct rwlock *rwl LOCK_FL_VARS
> > > WITNESS_UNLOCK(&rwl->rwl_lock_obj, wrlock ? LOP_EXCLUSIVE : 0,
> > > file, line);
> > >
> > > - membar_exit();
> > > + membar_exit_before_atomic();
> > > do {
> > > owner = rwl->rwl_owner;
> > > if (wrlock)
> > >
> > >
> >
>
