> 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...
> 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)
>
>
