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Date:   Wed, 21 Aug 2019 17:33:25 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc:     Will Deacon <will@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Joel Fernandes <joel@joelfernandes.org>,
        Alan Stern <stern@rowland.harvard.edu>,
        rostedt <rostedt@goodmis.org>,
        Valentin Schneider <valentin.schneider@arm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Will Deacon <will.deacon@arm.com>,
        David Howells <dhowells@redhat.com>
Subject: Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates
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On Wed, Aug 21, 2019 at 06:23:10AM -0700, Paul E. McKenney wrote:
> On Wed, Aug 21, 2019 at 11:32:01AM +0100, Will Deacon wrote:

> > and so it is using a store-pair instruction to reduce the complexity in
> > the immediate generation. Thus, the 64-bit store will only have 32-bit
> > atomicity. In fact, this is scary because if I change bar to:
> > 
> > void bar(u64 *x)
> > {
> > 	*(volatile u64 *)x = 0xabcdef10abcdef10;
> > }
> > 
> > then I get:
> > 
> > bar:
> > 	mov	w1, 61200
> > 	movk	w1, 0xabcd, lsl 16
> > 	str	w1, [x0]
> > 	str	w1, [x0, 4]
> > 	ret
> > 
> > so I'm not sure that WRITE_ONCE would even help :/
> 
> Well, I can have the LWN article cite your email, then.  So thank you
> very much!
> 
> Is generation of this code for a 64-bit volatile store considered a bug?
> Or does ARMv8 exclude the possibility of 64-bit MMIO registers?  And I
> would guess that Thomas and Linus would ask a similar bugginess question
> for normal stores.  ;-)

I'm calling this a compiler bug; the way I understand volatile this is
very much against the intentended use case. That is, this is buggy even
on UP vs signals or MMIO.