Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier

From: Waiman Long
Date: Fri Jun 17 2016 - 11:42:00 EST

On 06/16/2016 08:48 PM, Boqun Feng wrote:
On Thu, Jun 16, 2016 at 05:35:54PM -0400, Waiman Long wrote:
On 06/15/2016 10:19 PM, Boqun Feng wrote:
On Wed, Jun 15, 2016 at 03:01:19PM -0400, Waiman Long wrote:
On 06/15/2016 04:04 AM, Boqun Feng wrote:
Hi Waiman,

On Tue, Jun 14, 2016 at 06:48:04PM -0400, Waiman Long wrote:
The osq_lock() and osq_unlock() function may not provide the necessary
acquire and release barrier in some cases. This patch makes sure
that the proper barriers are provided when osq_lock() is successful
or when osq_unlock() is called.

Signed-off-by: Waiman Long<Waiman.Long@xxxxxxx>
---
kernel/locking/osq_lock.c | 4 ++--
1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 05a3785..7dd4ee5 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -115,7 +115,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
* cmpxchg in an attempt to undo our queueing.
*/

- while (!READ_ONCE(node->locked)) {
+ while (!smp_load_acquire(&node->locked)) {
/*
* If we need to reschedule bail... so we can block.
*/
@@ -198,7 +198,7 @@ void osq_unlock(struct optimistic_spin_queue *lock)
* Second most likely case.
*/
node = this_cpu_ptr(&osq_node);
- next = xchg(&node->next, NULL);
+ next = xchg_release(&node->next, NULL);
if (next) {
WRITE_ONCE(next->locked, 1);
So we still use WRITE_ONCE() rather than smp_store_release() here?

Though, IIUC, This is fine for all the archs but ARM64, because there
will always be a xchg_release()/xchg() before the WRITE_ONCE(), which
carries a necessary barrier to upgrade WRITE_ONCE() to a RELEASE.

Not sure whether it's a problem on ARM64, but I think we certainly need
to add some comments here, if we count on this trick.

Am I missing something or misunderstanding you here?

Regards,
Boqun
The change on the unlock side is more for documentation purpose than is
actually needed. As you had said, the xchg() call has provided the necessary
memory barrier. Using the _release variant, however, may have some
But I'm afraid the barrier doesn't remain if we replace xchg() with
xchg_release() on ARM64v8, IIUC, xchg_release() is just a ldxr+stlxr
loop with no barrier on ARM64v8. This means the following code:

CPU 0 CPU 1 (next)
======================== ==================
WRITE_ONCE(x, 1); r1 = smp_load_acquire(next->locked, 1);
xchg_release(&node->next, NULL); r2 = READ_ONCE(x);
WRITE_ONCE(next->locked, 1);

could result in (r1 == 1&& r2 == 0) on ARM64v8, IIUC.
If you look into the actual code:

next = xchg_release(&node->next, NULL);
if (next) {
WRITE_ONCE(next->locked, 1);
return;
}

There is a control dependency that WRITE_ONCE() won't happen until
But a control dependency only orders LOAD->STORE pairs, right? And here
the control dependency orders the LOAD part of xchg_release() and the
WRITE_ONCE().

Along with the fact that RELEASE only orders the STORE part of xchg with
the memory operations preceding the STORE part, so for the following
code:

WRTIE_ONCE(x,1);
next = xchg_release(&node->next, NULL);
if (next)
WRITE_ONCE(next->locked, 1);

such a reordering is allowed to happen on ARM64v8

next = ldxr [&node->next] // LOAD part of xchg_release()

if (next)
WRITE_ONCE(next->locked, 1);

WRITE_ONCE(x,1);
stlxr NULL [&node->next] // STORE part of xchg_releae()

Am I missing your point here?

Regards,
Boqun

My understanding of the release barrier is that both prior LOADs and STOREs can't move after the barrier. If WRITE_ONCE(x, 1) can move to below as shown above, it is not a real release barrier and we may need to change the barrier code.

Cheers,
Longman