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SubjectRe: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock()
On Mon, Jul 13, 2015 at 11:31:29PM +0100, Benjamin Herrenschmidt wrote:
> On Mon, 2015-07-13 at 13:15 +0100, Will Deacon wrote:
> > This didn't go anywhere last time I posted it, but here it is again.
> > I'd really appreciate some feedback from the PowerPC guys, especially as
> > to whether this change requires them to add an additional barrier in
> > arch_spin_unlock and what the cost of that would be.
>
> We'd have to turn the lwsync in unlock or the isync in lock into a full
> barrier. As it is, we *almost* have a full barrier semantic, but not
> quite, as in things can get mixed up inside spin_lock between the LL and
> the SC (things leaking in past LL and things leaking "out" up before SC
> and then getting mixed up in there).

Thanks, Ben.

> Michael, at some point you were experimenting a bit with that and tried
> to get some perf numbers of the impact that would have, did that
> solidify ? Otherwise, I'll have a look when I'm back next week.

These numbers would be really interesting...

Will


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