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

[Boqun Feng <boqun.feng@gmail.com>]

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On Fri, Jun 17, 2016 at 02:17:27PM -0400, Waiman Long wrote:
> 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

And sorry for the Red Pill ;-)

> 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?
> 

For PPC, I think xchg_release() + smp_store_release() is better than the 
current code, because the former has two lwsync while the latter has two
sync, and sync is quite expensive than lwsync on PPC.

I need to leave the ARM part to Will ;-)

Regards,
Boqun

> Cheers,
> Longman

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