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

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Message-ID: <20190904132418.GA237277@google.com>
Date:   Wed, 4 Sep 2019 09:24:18 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Alessio Balsini <balsini@...roid.com>, mingo@...nel.org,
        juri.lelli@...hat.com, linux-kernel@...r.kernel.org,
        dietmar.eggemann@....com, luca.abeni@...tannapisa.it,
        bristot@...hat.com, dvyukov@...gle.com, tglx@...utronix.de,
        vpillai@...italocean.com, kernel-team@...roid.com,
        will.deacon@....com
Subject: Re: [RFC][PATCH 01/13] sched/deadline: Impose global limits on
 sched_attr::sched_period

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@...radead.org> 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