linux-kernel - Re: [PATCH v2 2/6] lib/group_cpus: relax atomicity requirement in grp_spread_init

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Date:   Thu, 7 Dec 2023 18:49:20 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [PATCH v2 2/6] lib/group_cpus: relax atomicity requirement in
 grp_spread_init_one()

On Fri, Dec 08, 2023 at 09:31:27AM +0800, Ming Lei wrote:
> On Thu, Dec 07, 2023 at 12:38:56PM -0800, Yury Norov wrote:
> > Because nmsk and irqmsk are stable, extra atomicity is not required.
> > 
> > Signed-off-by: Yury Norov <yury.norov@...il.com>
> > ---
> >  lib/group_cpus.c | 9 ++++-----
> >  1 file changed, 4 insertions(+), 5 deletions(-)
> > 
> > diff --git a/lib/group_cpus.c b/lib/group_cpus.c
> > index ee272c4cefcc..8eb18c6bbf3b 100644
> > --- a/lib/group_cpus.c
> > +++ b/lib/group_cpus.c
> > @@ -24,8 +24,8 @@ static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk,
> >  		if (cpu >= nr_cpu_ids)
> >  			return;
> >  
> > -		cpumask_clear_cpu(cpu, nmsk);
> > -		cpumask_set_cpu(cpu, irqmsk);
> > +		__cpumask_clear_cpu(cpu, nmsk);
> > +		__cpumask_set_cpu(cpu, irqmsk);
> >  		cpus_per_grp--;
> >  
> >  		/* If the cpu has siblings, use them first */
> > @@ -34,9 +34,8 @@ static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk,
> >  			sibl = cpumask_next(sibl, siblmsk);
> >  			if (sibl >= nr_cpu_ids)
> >  				break;
> > -			if (!cpumask_test_and_clear_cpu(sibl, nmsk))
> > -				continue;
> > -			cpumask_set_cpu(sibl, irqmsk);
> > +			__cpumask_clear_cpu(sibl, nmsk);
> > +			__cpumask_set_cpu(sibl, irqmsk);
> >  			cpus_per_grp--;
> 
> Here the change isn't simply to remove atomicity, and the test
> part of cpumask_test_and_clear_cpu() is removed, so logic is changed,
> I feel the correct change should be:
> 
> 	if (cpumask_test_cpu(sibl, nmsk)) {
> 		__cpumask_clear_cpu(sibl, nmsk);
> 		__cpumask_set_cpu(sibl, irqmsk);
>   		cpus_per_grp--;
> 	}

Ohh. My mistake is that I put this patch prior to the #3, so people
bisecting the kernel may hit this problem... 

You're right here, but check the following patch: it switches the
for() loop to for_each_cpu_and_from(sibl, siblmsk, nmsk), and it means
that inside the loop sibl indexes set bits in both siblmsk and nmsk.

Now, because both masks are stable when the grp_spread_init_one() is
called, there's no chance to get nmks.sibl cleared suddenly, and it
means we can just drop the check.

Does this makes sense to you?

I can send v3 with a proper order of patches, if needed.

Thanks,
Yury