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

[Andrew Pinski]

On Fri, Dec 11, 2015 at 6:18 AM, Davidlohr Bueso wrote:
>
> 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?

Yes that works for me.  And yes that looks like the correct fix.

>>
>>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.

Yes as mutex_optimistic_spin calls into osq_lock/osq_unlock.  And
81a43adae3b9 changed mutex.c which David thought was where the issue
was located rather than not what mutex_optimistic_spin called.

Thanks,
Andrew Pinski

>
> Thanks,
> Davidlohr
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/