Re: [PATCH RT 10/30] hrtimer: Prevent using hrtimer_grab_expiry_lock() on migration_base

From: Tom Zanussi <zanussi@kernel.org>
To: Rasmus Villemoes <rasmus.villemoes@prevas.dk>,
	Steven Rostedt <rostedt@goodmis.org>,
	linux-kernel@vger.kernel.org,
	linux-rt-users <linux-rt-users@vger.kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Carsten Emde <C.Emde@osadl.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	John Kacur <jkacur@redhat.com>, Daniel Wagner <wagi@monom.org>,
	Julien Grall <julien.grall@arm.com>
Subject: Re: [PATCH RT 10/30] hrtimer: Prevent using hrtimer_grab_expiry_lock() on migration_base
Date: Tue, 28 Apr 2020 08:43:10 -0500	[thread overview]
Message-ID: <a63a1c2aae7dca43334ff3b72c17adeefade6c8e.camel@kernel.org> (raw)
In-Reply-To: <049616fa-8908-e845-057f-d9482a483597@prevas.dk>

On Tue, 2020-04-28 at 15:07 +0200, Rasmus Villemoes wrote:
> On 28/04/2020 14.59, Tom Zanussi wrote:
> > On Tue, 2020-04-28 at 09:03 +0200, Rasmus Villemoes wrote:
> > > Hold on a second. This patch (hrtimer: Prevent using
> > > hrtimer_grab_expiry_lock() on migration_base) indeed seems to
> > > implement
> > > the optimization implied by the above, namely avoid the
> > > lock/unlock
> > > in
> > > case base == migration_base:
> > > 
> > > > -	if (timer->is_soft && base && base->cpu_base) {
> > > > +	if (timer->is_soft && base != &migration_base) {
> > > 
> > > But the followup patch (hrtimer: Add a missing bracket and hide
> > > `migration_base on !SMP) to fix the build on !SMP [the missing
> > > bracket
> > > part seems to have been fixed when backporting the above to 4.19-
> > > rt]
> > > replaces that logic by
> > > 
> > > +static inline bool is_migration_base(struct hrtimer_clock_base
> > > *base)
> > > +{
> > > +	return base == &migration_base;
> > > +}
> > > +
> > > ...
> > > -	if (timer->is_soft && base != &migration_base) {
> > > +	if (timer->is_soft && is_migration_base(base)) {
> > > 
> > > in the SMP case, i.e. the exact opposite condition. One of these
> > > can't
> > > be correct.
> > > 
> > > Assuming the followup patch was wrong and the condition should
> > > have
> > > read
> > > 
> > >   timer->is_soft && !is_migration_base(base)
> > > 
> > > while keeping is_migration_base() false on !SMP might explain the
> > > problem I see. But I'd like someone who knows this code to chime
> > > in.
> > > 
> > 
> > I don't know this code, but I think you're correct - the followup
> > patch
> > reversed the condition by forgetting the !.
> > 
> > So, does your problem go away when you make that change?
> 
> Yes, it does. (I'll have to ask the customer to check in their setup
> whether the boot hang also vanishes).
> 
> Essentially, adding that ! is equivalent to reverting the two patches
> on
> !SMP (which I also tested): Before, the condition was
> 
>   timer->is_soft && base && base->cpu_base
> 
> and, assuming the NULL pointer checks are indeed redundant, that's
> the
> same as "timer->is_soft". Appending " && !is_migration_base()" to
> that,
> with is_migration_base() always false as on !SMP, doesn't change
> anything.
> 

OK, great, thanks for tracking this down.

If you post a patch that makes that change and mention that it's a fix
for commit "40aae5708e7a hrtimer: Add a missing bracket and hide
`migration_base' on !SMP", I can pull it into a new update release.

Thanks,

Tom

> Thanks,
> Rasmus