Re: Linux 3.19-rc3

Peter Zijlstra
2015-01-06T12:20:24Z

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.