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Date:   Mon, 2 Sep 2019 14:31:06 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Alessio Balsini <balsini@android.com>
Cc:     mingo@kernel.org, juri.lelli@redhat.com,
        linux-kernel@vger.kernel.org, dietmar.eggemann@arm.com,
        luca.abeni@santannapisa.it, bristot@redhat.com, dvyukov@google.com,
        tglx@linutronix.de, vpillai@digitalocean.com, rostedt@goodmis.org,
        kernel-team@android.com
Subject: Re: [RFC][PATCH 01/13] sched/deadline: Impose global limits on
 sched_attr::sched_period
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On Mon, Sep 02, 2019 at 11:16:23AM +0200, Peter Zijlstra wrote:
> On Sat, Aug 31, 2019 at 03:41:17PM +0100, Alessio Balsini wrote:
> > Right!
> > 
> > Verified that sysctl_sched_dl_period_max and sysctl_sched_dl_period_min values
> > are now always consistent.
> > 
> > I spent some time in trying to figure out if not having any mutex in
> > __checkparam_dl() is safe. There can surely happen that "max < min", e.g.:

> > Sharing my thoughts, a "BUG_ON(max < min)" in __checkparam_dl() is then a
> > guaranteed source of explosions, but the good news is that "if (period < min ||
> > period > max" in __checkparam_dl() surely fails if "max < min".  Also the fact
> > that, when we are writing the new sysctl_sched_dl_* values, only one is
> > actually changed at a time, that surely helps to preserve the consistency.
> > 
> > But is that enough?
> 
> Strictly speaking, no, I suppose it is not. We can have two changes in
> between the two READ_ONCE()s and then we'd be able to observe a
> violation.
> 
> The easy way to fix that is do something like:
> 
> +	synchronize_rcu();
> 	mutex_unlock(&mutex);
> 
> 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();
> 
> in __checkparam_dl().
> 
> That would prohibit we see two changes, and seeing only the single
> change is safe.

I pushed out a new version; and added patch to sched_rt_handler() on
top.

Please have a look at:

  git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git sched/wip-deadline

I'll move these two patches to sched/core if everything looks good.