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Re: [RFC][PATCH v3]: documentation,atomic: Add new documents

From Will Deacon <will.deacon@arm.com>
Newsgroups linux.kernel
Subject Re: [RFC][PATCH v3]: documentation,atomic: Add new documents
Date 2017-08-01 12:20 +0200
Message-ID <u9Cro-2tI-9@gated-at.bofh.it> (permalink)
References (5 earlier) <u7tVg-mT-9@gated-at.bofh.it> <u9eS6-4nS-11@gated-at.bofh.it> <u9gKd-5wZ-7@gated-at.bofh.it> <u9mZj-Kh-9@gated-at.bofh.it> <u9BlE-1r3-25@gated-at.bofh.it>
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On Tue, Aug 01, 2017 at 11:01:21AM +0200, Peter Zijlstra wrote:
> On Mon, Jul 31, 2017 at 10:43:45AM -0700, Paul E. McKenney wrote:
> 
> > Why wouldn't the following have ACQUIRE semantics?
> > 
> > 	atomic_inc(&var);
> > 	smp_mb__after_atomic();
> > 
> > Is the issue that there is no actual value returned or some such?
> 
> Yes, so that the inc is a load-store, and thus there is a load, we loose
> the value.
> 
> But I see your point I think. Irrespective of still having the value,
> the ordering is preserved and nothing should pass across that.
> 
> > So if I have something like this, the assertion really can trigger?
> > 
> > 	WRITE_ONCE(x, 1);		atomic_inc(&y);
> > 	r0 = xchg_release(&y, 5);	smp_mb__after_atomic();
> > 					r1 = READ_ONCE(x);
> > 
> > 
> > 	WARN_ON(r0 == 0 && r1 == 0);
> > 
> > I must confess that I am not seeing why we would want to allow this
> > outcome.
> 
> No you are indeed quite right. I just wasn't creative enough. Thanks for
> the inspiration.

Just to close this out, we agree that an smp_rmb() instead of
smp_mb__after_atomic() would *not* forbid this outcome, right?

Will

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Re: [RFC][PATCH v3]: documentation,atomic: Add new documents "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-31 19:50 +0200
  Re: [RFC][PATCH v3]: documentation,atomic: Add new documents Boqun Feng <boqun.feng@gmail.com> - 2017-08-01 04:20 +0200
  Re: [RFC][PATCH v3]: documentation,atomic: Add new documents Peter Zijlstra <peterz@infradead.org> - 2017-08-01 11:10 +0200
    Re: [RFC][PATCH v3]: documentation,atomic: Add new documents Will Deacon <will.deacon@arm.com> - 2017-08-01 12:20 +0200
      Re: [RFC][PATCH v3]: documentation,atomic: Add new documents Peter Zijlstra <peterz@infradead.org> - 2017-08-01 13:50 +0200
        Re: [RFC][PATCH v3]: documentation,atomic: Add new documents Will Deacon <will.deacon@arm.com> - 2017-08-01 14:20 +0200
          Re: [RFC][PATCH v3]: documentation,atomic: Add new documents Peter Zijlstra <peterz@infradead.org> - 2017-08-01 15:00 +0200
          Re: [RFC][PATCH v3]: documentation,atomic: Add new documents "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-08-01 18:20 +0200
            Re: [RFC][PATCH v3]: documentation,atomic: Add new documents Peter Zijlstra <peterz@infradead.org> - 2017-08-01 18:50 +0200
              Re: [RFC][PATCH v3]: documentation,atomic: Add new documents Will Deacon <will.deacon@arm.com> - 2017-08-01 19:00 +0200
              Re: [RFC][PATCH v3]: documentation,atomic: Add new documents "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-08-02 00:20 +0200
                Re: [RFC][PATCH v3]: documentation,atomic: Add new documents Will Deacon <will.deacon@arm.com> - 2017-08-02 10:50 +0200
            Re: [RFC][PATCH v3]: documentation,atomic: Add new documents "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-08-01 20:40 +0200
            Re: [RFC][PATCH v3]: documentation,atomic: Add new documents Will Deacon <will.deacon@arm.com> - 2017-08-02 11:50 +0200
              Re: [RFC][PATCH v3]: documentation,atomic: Add new documents "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-08-02 18:20 +0200
              Re: [RFC][PATCH v3]: documentation,atomic: Add new documents Boqun Feng <boqun.feng@gmail.com> - 2017-08-03 16:10 +0200
                Re: [RFC][PATCH v3]: documentation,atomic: Add new documents "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-08-03 17:00 +0200
                Re: [RFC][PATCH v3]: documentation,atomic: Add new documents Will Deacon <will.deacon@arm.com> - 2017-08-03 18:20 +0200
                Re: [RFC][PATCH v3]: documentation,atomic: Add new documents "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-08-03 19:10 +0200
    Re: [RFC][PATCH v3]: documentation,atomic: Add new documents "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-08-01 15:40 +0200

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