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

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Date:	Fri, 11 Dec 2015 06:17:47 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Will Deacon <will.deacon@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Andrew Pinski <andrew.pinski@...iumnetworks.com>,
	Davidlohr Bueso <dbueso@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: FW: Commit 81a43adae3b9 (locking/mutex: Use acquire/release
 semantics) causing failures on arm64 (ThunderX)

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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