Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates

[Will Deacon]

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:
> > 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?

I consider it a bug for the volatile case, and the one compiler person I've
spoken to also seems to reckon it's a bug, so hopefully it will be fixed.
I'm led to believe it's an optimisation in the AArch64 backend of GCC.

> 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.  ;-)

We use inline asm for MMIO, fwiw.

Will