Messages in this thread | | | Date | Tue, 6 Jan 2015 13:20:06 +0100 | From | Peter Zijlstra <> | Subject | Re: Linux 3.19-rc3 |
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On Tue, Jan 06, 2015 at 04:01:21AM -0800, Kent Overstreet wrote: > On Tue, Jan 06, 2015 at 12:48:42PM +0100, Peter Zijlstra wrote: > > > > > > Looking at that closure stuff, why is there an smp_mb() in > > closure_wake_up() ? Typically wakeup only needs to imply a wmb. > > > > Also note that __closure_wake_up() starts with a fully serializing > > instruction (xchg) and thereby already implies the full barrier. > > Probably no good reason, that code is pretty old :) > > If I was to hazard a guess, I had my own lockless linked lists before llist.h > existed and perhaps I did it with atomic_xchg() - which was at least documented > to not imply a barrier. I suppose it should just be dropped.
We (probably me) should probably audit all the atomic_xchg() implementations and documentation and fix that. I was very much under the impression it should imply a full barrier (and it certainly does on x86), the documentation should state the rule that any atomic_ function that returns a result is fully serializing, therefore, because atomic_xchg() has a return value, it should too.
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