linux-kernel - Re: [PATCH RT 10/30] hrtimer: Prevent using hrtimer_grab_expiry_lock() on migration

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Date:   Tue, 28 Apr 2020 07:59:26 -0500
From:   Tom Zanussi <zanussi@...nel.org>
To:     Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org,
        linux-rt-users <linux-rt-users@...r.kernel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        John Kacur <jkacur@...hat.com>, Daniel Wagner <wagi@...om.org>,
        Julien Grall <julien.grall@....com>
Subject: Re: [PATCH RT 10/30] hrtimer: Prevent using
 hrtimer_grab_expiry_lock() on migration_base

On Tue, 2020-04-28 at 09:03 +0200, Rasmus Villemoes wrote:
> On 23/01/2020 21.39, Steven Rostedt wrote:
> > 4.19.94-rt39-rc2 stable review patch.
> > If anyone has any objections, please let me know.
> > 
> > ------------------
> > 
> > From: Julien Grall <julien.grall@....com>
> > 
> > [ Upstream commit cef1b87f98823af923a386f3f69149acb212d4a1 ]
> > 
> > As tglx puts it:
> > > If base == migration_base then there is no point to lock
> > > soft_expiry_lock
> > > simply because the timer is not executing the callback in soft
> > > irq context
> > > and the whole lock/unlock dance can be avoided.
> 
> 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?

Tom

> Thanks,
> Rasmus