Re: FW: Commit 81a43adae3b9 (locking/mutex: Use acquire/release semantics) causing failures on arm64 (ThunderX)

[Paul E. McKenney]

On Mon, Dec 14, 2015 at 09:28:55PM +0100, Peter Zijlstra wrote:
> On Fri, Dec 11, 2015 at 02:35:40PM -0800, Paul E. McKenney wrote:
> > 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.
> > 
> > I am probably missing quite a bit on this thread, but don't x86 MMIO
> > accesses to frame buffers need to interact with something more heavyweight
> > than an x86 release store or acquire load in order to remain confined
> > to the resulting critical section?
> 
> So on x86 there really isn't a problem because every atomic op (and
> there's plenty here) will be a full barrier.
> 
> That is, even if you were to replace everything with _relaxed() ops, it
> would still work as 'expected' on x86.
> 
> ppc/arm64 will crash and burn, but that's another story.
> 
> But the important point here was that osq_wait_next() is never relied
> upon to provide either the ACQUIRE semantics for osq_lock() not the
> RELEASE semantics for osq_unlock(). Those are provided by other ops.

OK, good to know!

							Thanx, Paul

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