Messages in this thread | | | Date | Thu, 25 Feb 2016 15:48:38 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores |
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On Thu, Feb 25, 2016 at 06:07:03AM -0800, Paul E. McKenney wrote: > Still bad wording... > > It hasn't actually moved anything over the barrier(). It has instead > moved both the barrier() and the WRITE_ONCE(b, p) to precede the "if > (q)". Mathieu mentioned this over IRC yesterday, and I queue a change > so that the paragraph now reads as follows: > > (*) If both legs of the "if" statement begin with identical stores to > the same variable, then those stores must be ordered, either by > preceding both of them with smp_mb() or by using smp_store_release() > to carry out the stores. Please note that it is -not- sufficient > to use barrier() at beginning of each leg of the "if" statement > because, as shown by the example above, optimizing compilers can > destroy the control dependency while respecting the letter of the > barrier() law. > > Does hat help?
Maybe.. I still feel the compiler should not do this; but I'm having a hard time explaining why.
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