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SubjectRe: FW: Commit 81a43adae3b9 (locking/mutex: Use acquire/release semantics) causing failures on arm64 (ThunderX)
On Fri, Dec 11, 2015 at 02:48:03PM +0100, Peter Zijlstra wrote:
> On Fri, Dec 11, 2015 at 01:33:14PM +0000, Will Deacon wrote:
> > On Fri, Dec 11, 2015 at 01:26:47PM +0100, Peter Zijlstra wrote:
>
> > > While we're there, the acquire in osq_wait_next() seems somewhat ill
> > > documented too.
> > >
> > > I _think_ we need ACQUIRE semantics there because we want to strictly
> > > order the lock-unqueue A,B,C steps and we get that with:
> > >
> > > A: SC
> > > B: ACQ
> > > C: Relaxed
> > >
> > > Similarly for unlock we want the WRITE_ONCE to happen after
> > > osq_wait_next, but in that case we can even rely on the control
> > > dependency there.
> >
> > Even for the lock-unqueue case, isn't B->C ordered by a control dependency
> > because C consists only of stores?
>
> Hmm, indeed. So we could go fully relaxed on it I suppose, since the
> same is true for the unlock site.

In which case, we should be able to relax the xchg in there (osq_wait_next)
too, right?

Will


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