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

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Message-ID: <20151214203154.GY6357@twins.programming.kicks-ass.net>
Date:	Mon, 14 Dec 2015 21:31:54 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Will Deacon <will.deacon@....com>,
	Andrew Pinski <andrew.pinski@...iumnetworks.com>,
	Davidlohr Bueso <dbueso@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Commit 81a43adae3b9 (locking/mutex: Use acquire/release
 semantics) causing failures on arm64 (ThunderX)

On Mon, Dec 14, 2015 at 06:49:31PM +0000, One Thousand Gnomes wrote:
> 
> But seriously are there any cases we actually care about this for osq ?

So I think what PaulMck is worried about is that one would expect things
like:

	mutex_lock();
	MMIO(++var);
	mutex_unlock();

(the same example Linus gave but with a mutex instead of a spinlock) to
just work.

Now, I haven't checked the code, but I'm not sure we ever rely on osq to
provide the mutex ACQUIRE barrier, since we always need to acquire the
mutex variable itself after we've acquired the osq 'lock'.

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