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

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Message-ID: <ZXKNVRu3AfvjaFhK@fedora>
Date:   Fri, 8 Dec 2023 11:28:21 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Yury Norov <yury.norov@...il.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 Thu, Dec 07, 2023 at 06:49:20PM -0800, Yury Norov wrote:
> 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.

v3 is correct, and I'd suggest to either fix v2 or re-order v3,
otherwise both patch 2 and 3 are not easy to follow.


Thanks,
Ming