Messages in this thread | | | Date | Tue, 20 Feb 2018 08:11:10 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] tools/memory-model: remove rb-dep, smp_read_barrier_depends, and lockless_dereference |
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On Tue, Feb 20, 2018 at 04:17:52PM +0100, Andrea Parri wrote: > On Tue, Feb 20, 2018 at 06:48:13AM -0800, Paul E. McKenney wrote: > > On Mon, Feb 19, 2018 at 06:44:13PM +0100, Peter Zijlstra wrote: > > > On Mon, Feb 19, 2018 at 12:14:45PM -0500, Alan Stern wrote: > > > > Note that operations like atomic_add_unless() already include memory > > > > barriers. > > > > > > It is valid for atomic_add_unless() to not imply any barriers when the > > > addition doesn't happen. > > > > Agreed, given that atomic_add_unless() just returns 0 or 1, not the > > pointer being added. Of course, the __atomic_add_unless() function > > that it calls is another story, as it does return the old value. Sigh. > > And __atomic_add_unless() is called directly from some code. All of > > which looks to be counters rather than pointers, thankfully. > > > > So, do we want to rely on atomic_add_unless() always being > > invoked on counters rather than pointers, or does it need an > > smp_read_barrier_depends()? > > alpha's implementation of __atomic_add_unless() has an unconditional smp_mb() > before returning so, as far as dependencies are concerned, these seem fine.
Very good!
Thanx, Paul
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