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

From: [email-masked] (Davidlohr Bueso)
Date: Fri, 11 Dec 2015 06:17:47 -0800

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