Date: Fri, 11 Dec 2015 06:17:47 -0800
From: Davidlohr Bueso <dave@stgolabs.net>
To: Will Deacon <will.deacon@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
        Andrew Pinski <andrew.pinski@caviumnetworks.com>,
        Davidlohr Bueso <dbueso@suse.de>, Thomas Gleixner <tglx@linutronix.de>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: FW: Commit 81a43adae3b9 (locking/mutex: Use acquire/release
 semantics) causing failures on arm64 (ThunderX)
Message-ID: <20151211141747.GC5650@linux-uzut.site>
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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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