On 06/17/2016 11:45 AM, Will Deacon wrote:
On Fri, Jun 17, 2016 at 11:26:41AM -0400, Waiman Long wrote:
On 06/16/2016 08:48 PM, Boqun Feng wrote:
On Thu, Jun 16, 2016 at 05:35:54PM -0400, Waiman Long wrote:
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?
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.
You seem to be missing the point.
{READ,WRITE}_ONCE accesses appearing in program order after a release
are not externally ordered with respect to the release unless they
access the same location.
This is illustrated by Boqun's example, which shows two WRITE_ONCE
accesses being reordered before a store-release forming the write
component of an xchg_release. In both cases, WRITE_ONCE(x, 1) remains
ordered before the store-release.
Will
I am sorry that I misread the mail. I am not used to treating xchg as
two separate instructions. Yes, it is a problem. In that case, we have
to either keep the xchg() function as it is or use
smp_store_release(&next->locked, 1). So which one is a better
alternative for ARM or PPC?
Cheers,
Longman
--
To unsubscribe from this list: send the line "unsubscribe linux-arch" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
