Messages in this thread | | | Date | Tue, 06 Jan 2015 07:55:39 -0500 | From | Peter Hurley <> | Subject | Re: Linux 3.19-rc3 |
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[ +cc Paul McKenney ]
On 01/06/2015 07:20 AM, Peter Zijlstra wrote: > 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.
memory-barriers.txt and atomic_ops.txt appear to contradict each other here, but I think that's because atomic_ops.txt has drifted toward an arch-implementer's POV:
260:atomic_xchg requires explicit memory barriers around the operation.
All the serializing atomic operations have descriptions like this.
Regards, Peter Hurley
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