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

[Davidlohr Bueso]

On Fri, 11 Dec 2015, Will Deacon wrote:

> I think Andrew meant the atomic_xchg_acquire at the start of osq_lock,
> as opposed to "compare and swap". In which case, it does look like
> there's a bug here because there is nothing to order the initialisation
> of the node fields with publishing of the node, whether that's
> indirectly as a result of setting the tail to the current CPU or
> directly as a result of the WRITE_ONCE.

Sorry I'm late to the party.

Duh yes this is obviously bogus, and worse I recall triggering a similar
tail initialization issue in osq_lock on some experimental work on x86,
so this is very much a point of failure. Ack.

> Andrew, David: does making that atomic_xchg_acquire and atomic_xchg
> fix things for you?
>
> I don't fully grok what 81a43adae3b9 has to do with any of this, so
> maybe there's another bug too.

I think this is mainly because mutex_optimistic_spin is where the stack
shows the lockup, which really translates to c55a6ffa62.

Thanks,
Davidlohr
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