Re: [RFC][PATCH 01/13] sched/deadline: Impose global limits on sched_attr::sched_period

[Joel Fernandes]

On Wed, Sep 04, 2019 at 01:30:38PM +0200, Peter Zijlstra wrote:
> On Wed, Sep 04, 2019 at 06:16:16AM -0400, Steven Rostedt wrote:
> > On Mon, 2 Sep 2019 11:16:23 +0200
> > Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > 
> > > in sched_dl_period_handler(). And do:
> > > 
> > > +	preempt_disable();
> > > 	max = (u64)READ_ONCE(sysctl_sched_dl_period_max) * NSEC_PER_USEC;
> > > 	min = (u64)READ_ONCE(sysctl_sched_dl_period_min) * NSEC_PER_USEC;
> > > +	preempt_enable();
> > 
> > Hmm, I'm curious. Doesn't the preempt_disable/enable() also add
> > compiler barriers which would remove the need for the READ_ONCE()s here?
> 
> They do add compiler barriers; but they do not avoid the compiler
> tearing stuff up.

Neither does WRITE_ONCE() on some possibly buggy but currently circulating
compilers :(

As Will said in:
https://lore.kernel.org/lkml/20190821103200.kpufwtviqhpbuv2n@willie-the-truck/

void bar(u64 *x)
{
	*(volatile u64 *)x = 0xabcdef10abcdef10;
}

gives:

bar:
	mov	w1, 61200
	movk	w1, 0xabcd, lsl 16
	str	w1, [x0]
	str	w1, [x0, 4]
	ret

Speaking of which, Will, is there a plan to have compiler folks address this
tearing issue and are bugs filed somewhere? I believe aarch64 gcc is buggy,
and clang is better but is still buggy?

thanks,

 - Joel