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Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep

From "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Newsgroups linux.kernel
Subject Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep
Date 2016-06-05 06:40 +0200
Message-ID <rGy0W-2a4-1@gated-at.bofh.it> (permalink)
References (5 earlier) <rFTKa-1on-7@gated-at.bofh.it> <rFW5k-2Y2-33@gated-at.bofh.it> <rFWoF-36Q-9@gated-at.bofh.it> <rFXuq-3Iu-27@gated-at.bofh.it> <rFXE6-3Ng-41@gated-at.bofh.it>
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On Fri, Jun 03, 2016 at 02:45:53PM +0100, Will Deacon wrote:
> On Fri, Jun 03, 2016 at 06:32:38AM -0700, Paul E. McKenney wrote:
> > On Fri, Jun 03, 2016 at 02:23:10PM +0200, Peter Zijlstra wrote:
> > > On Fri, Jun 03, 2016 at 05:08:27AM -0700, Paul E. McKenney wrote:
> > > > On Fri, Jun 03, 2016 at 11:38:34AM +0200, Peter Zijlstra wrote:
> > > > > On Fri, Jun 03, 2016 at 02:48:38PM +0530, Vineet Gupta wrote:
> > > > > > On Wednesday 25 May 2016 09:27 PM, Paul E. McKenney wrote:
> > > > > > > For your example, but keeping the compiler in check:
> > > > > > > 
> > > > > > > 	if (READ_ONCE(a))
> > > > > > > 		WRITE_ONCE(b, 1);
> > > > > > > 	smp_rmb();
> > > > > > > 	WRITE_ONCE(c, 2);
> > > > > 
> > > > > So I think it example is broken. The store to @c is not in fact
> > > > > dependent on the condition of @a.
> > > > 
> > > > At first glance, the compiler could pull the write to "c" above the
> > > > conditional, but the "memory" constraint in smp_rmb() prevents this.
> > > > From a hardware viewpoint, the write to "c" does depend on the "if",
> > > > as the conditional branch does precede that write in execution order.
> > > > 
> > > > But yes, this is using smp_rmb() in a very strange way, if that is
> > > > what you are getting at.
> > > 
> > > Well, the CPU could decide that the store to C happens either way around
> > > the branch. I'm not sure I'd rely on CPUs not being _that_ clever.
> > 
> > If I remember correctly, both Power and ARM guarantee that the CPU won't
> > be that clever.  Not sure about Itanium.
> 
> I wouldn't be so sure about ARM. On 32-bit, at least, we have conditional
> store instructions so if the compiler could somehow use one of those for
> the first WRITE_ONCE then there's very obviously no control dependency
> on the second WRITE_ONCE and they could be observed out of order.

OK, good to know...

> I note that smp_rmb() on ARM and arm64 actually orders against subsequent
> (in program order) writes, so this is still pretty theoretical for us.

So the combined control-dependency/smp_rmb() still works, but I should
re-examine the straight control dependency stuff.

								Thanx, Paul

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Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2016-06-03 11:20 +0200
  Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep Peter Zijlstra <peterz@infradead.org> - 2016-06-03 11:40 +0200
    Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-03 14:10 +0200
      Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep Peter Zijlstra <peterz@infradead.org> - 2016-06-03 14:30 +0200
        Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep Peter Zijlstra <peterz@infradead.org> - 2016-06-03 14:30 +0200
          Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-03 15:40 +0200
        Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-03 15:40 +0200
          Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep Will Deacon <will.deacon@arm.com> - 2016-06-03 15:50 +0200
            Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-05 06:40 +0200
              Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-06 19:30 +0200
                Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep Peter Zijlstra <peterz@infradead.org> - 2016-06-07 09:20 +0200
                Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep Hannes Frederic Sowa <hannes@stressinduktion.org> - 2016-06-07 14:50 +0200
                Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-07 15:10 +0200
                Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep Hannes Frederic Sowa <hannes@stressinduktion.org> - 2016-06-07 17:00 +0200
                Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-07 17:30 +0200
                Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep Peter Zijlstra <peterz@infradead.org> - 2016-06-07 19:50 +0200
                Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-07 20:50 +0200
                Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep Will Deacon <will.deacon@arm.com> - 2016-06-07 20:10 +0200
                Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-07 20:50 +0200
                Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-06-07 21:00 +0200
                Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep Hannes Frederic Sowa <hannes@stressinduktion.org> - 2016-06-07 20:40 +0200

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