linux-kernel - Re: [RFC][PATCH 7/7] kref: Implement using refcount

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Date:   Fri, 18 Nov 2016 10:16:26 +0000
From:   Will Deacon <will.deacon@....com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, gregkh@...uxfoundation.org,
        keescook@...omium.org, elena.reshetova@...el.com, arnd@...db.de,
        tglx@...utronix.de, mingo@...nel.org, hpa@...or.com,
        dave@...gbits.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 7/7] kref: Implement using refcount_t

On Fri, Nov 18, 2016 at 04:26:34PM +0800, Boqun Feng wrote:
> On Thu, Nov 17, 2016 at 04:36:24PM +0000, Will Deacon wrote:
> > On Thu, Nov 17, 2016 at 05:11:10PM +0100, Peter Zijlstra wrote:
> > > On Thu, Nov 17, 2016 at 12:08:36PM +0000, Will Deacon wrote:
> > > > All sounds reasonable to me. It's worth pointing out that you can't create
> > > > order using a control dependency hanging off the status flag of a
> > > > store-conditional, but the code in question here has the dependency from
> > > > the loaded value, which is sufficient.
> > > 
> > > Yeah, I'm always surprised by that 'feature'. Is that ARM specific? Or
> > > so more LL/SC archs have this?
> > 
> > In general, I'm not sure, but I think PPC does allow for the control
> > dependency.
> > 
> 
> You guys mean the "control dependency" from a sc to subsequent WRITE,
> like in the following litmus?
> 
> 	PPC sc-control
> 	""
> 	{
> 	0:r11=x;0:r12=y;0:r3=1;0:r10=0;
> 	1:r11=x;1:r12=y;
> 	}
> 
> 	P0                   | P1             ;
> 	lwarx r2, r10, r11   | lwz r2, 0(r12) ;
> 	stwcx. r3, r10, r11  | lwsync           ;
> 	bne Fail             |                ;
> 	stw r3, 0(r12)       | lwz r1, 0(r11) ;
> 	Fail:                |                ;
> 
> 	exists
> 	(1:r2 = 1 /\ x = 1 /\ 1:r1 = 0)
> 
> PPCMEM and herd both said the exists-clause could be triggered
> "Sometimes".
> 
> And ISA said:
> 
> """
> Because a Store Conditional instruction may complete before its store
> has been performed, a conditional Branch instruction that depends on the
> CR0 value set by a Store Conditional instruction does not order the
> Store Conditional's store with respect to storage accesses caused by
> instructions that follow the Branch.
> """
> 
> So ppc doesn't honor this "control dependency". ;-)

Oh, wow! I was feeling like the odd duck after talking to paulmck and
Alan Stern about this at LPC, but now it looks like I have company on
the island of misfit memory models.

Will