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

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>,
	Andrew Pinski <andrew.pinski@caviumnetworks.com>,
	Davidlohr Bueso <dbueso@suse.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	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>,
	david.daney@cavium.com
Subject: Re: FW: Commit 81a43adae3b9 (locking/mutex: Use acquire/release semantics) causing failures on arm64 (ThunderX)
Date: Fri, 11 Dec 2015 14:35:40 -0800	[thread overview]
Message-ID: <20151211223540.GA22277@linux.vnet.ibm.com> (raw)
In-Reply-To: <20151211134803.GP6356@twins.programming.kicks-ass.net>

On Fri, Dec 11, 2015 at 02:48:03PM +0100, Peter Zijlstra wrote:
> On Fri, Dec 11, 2015 at 01:33:14PM +0000, Will Deacon wrote:
> > On Fri, Dec 11, 2015 at 01:26:47PM +0100, Peter Zijlstra wrote:
> 
> > > While we're there, the acquire in osq_wait_next() seems somewhat ill
> > > documented too.
> > > 
> > > I _think_ we need ACQUIRE semantics there because we want to strictly
> > > order the lock-unqueue A,B,C steps and we get that with:
> > > 
> > >  A: SC
> > >  B: ACQ
> > >  C: Relaxed
> > > 
> > > Similarly for unlock we want the WRITE_ONCE to happen after
> > > osq_wait_next, but in that case we can even rely on the control
> > > dependency there.
> > 
> > Even for the lock-unqueue case, isn't B->C ordered by a control dependency
> > because C consists only of stores?
> 
> Hmm, indeed. So we could go fully relaxed on it I suppose, since the
> same is true for the unlock site.

I am probably missing quite a bit on this thread, but don't x86 MMIO
accesses to frame buffers need to interact with something more heavyweight
than an x86 release store or acquire load in order to remain confined
to the resulting critical section?

							Thanx, Paul