linux-kernel - Re: [PATCH 2/2] [PATCH] sched: Add smp

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Date:	Tue, 17 Feb 2015 19:23:35 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Kirill Tkhai <ktkhai@...allels.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>, oleg@...hat.com
Subject: Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking
 cycles

On Tue, Feb 17, 2015 at 10:01:54AM -0800, Paul E. McKenney wrote:
> > > The blob under SMP BARRIER PAIRING does not mention pairing with control
> > > dependencies; and I'm rather sure I've done so.
> 
> And here is a patch for the control-dependency pairing.  Thoughts?

The proposed patch does not change the blub under SMP BARRIER PAIRING,
which would be the first place I would look for this.

> @@ -850,6 +853,19 @@ Or:
>  			      <data dependency barrier>
>  			      y = *x;
>  
> +Or even:
> +
> +	CPU 1		      CPU 2
> +	===============	      ===============================
> +	r1 = ACCESS_ONCE(y);
> +	<write barrier>
> +	ACCESS_ONCE(y) = 1;   if (r2 = ACCESS_ONCE(x)) {
> +			         <implicit control dependency>
> +			         ACCESS_ONCE(y) = 1;
> +			      }
> +
> +	assert(r1 == 0 || r2 == 0);
> +
>  Basically, the read barrier always has to be there, even though it can be of
>  the "weaker" type.

Does that want to be a <general barrier>; CPU1 looks to do a LOAD
followed by a STORE, separated by a WMB, which is of course odd.

To me the pairing with a general barrier is also the most intuitive,
since the control dependency is a LOAD -> STORE order.

Then again, I'm rather tired and maybe I'm missing the obvious :-)
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