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

[One Thousand Gnomes]

On Fri, 11 Dec 2015 14:35:40 -0800
"Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> 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?

Depends upon the device and the mapping. There are also CPU errata
related to write combining on older CPUs (notably Pentium Pro era) which
result in ordering errors with write combining unless deliberately fenced.

Any PCI access isn't constrained to the critical section unless a PCI
read from the same device is done and completes before exiting. Even then
on processors with a separate APIC bus (PPro, PII I think) interrupts are
asynchronous on their own bus.

The PCI posting rules also apply to DMA.

Finally we run the IDT WinChip in out-of-order store mode not full x86
compatibility which while uniprocessor does mean the correct fences
matter.

Just to ensure total confusion some video cards have MMIO areas that are
not in fact memory but a FIFO rigged to look like a block of RAM for
speed of writing. In those cases the rules are a bit card dependant.

But seriously are there any cases we actually care about this for osq ?

Alan

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