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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
