linux-kernel - Re: [PATCH 09/33] block: Protect against concurrent isolated cpuset change

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Message-ID: <aVUs-mFYCO2qGmqT@localhost.localdomain>
Date: Wed, 31 Dec 2025 15:02:34 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Jens Axboe <axboe@...nel.dk>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Michal Koutný <mkoutny@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Chen Ridong <chenridong@...wei.com>,
	Danilo Krummrich <dakr@...nel.org>,
	"David S . Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Gabriele Monaco <gmonaco@...hat.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ingo Molnar <mingo@...hat.com>, Jakub Kicinski <kuba@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
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	Michal Hocko <mhocko@...e.com>, Muchun Song <muchun.song@...ux.dev>,
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	netdev@...r.kernel.org
Subject: Re: [PATCH 09/33] block: Protect against concurrent isolated cpuset
 change

Le Mon, Dec 29, 2025 at 05:37:29PM -0700, Jens Axboe a écrit :
> On 12/24/25 6:44 AM, Frederic Weisbecker wrote:
> > The block subsystem prevents running the workqueue to isolated CPUs,
> > including those defined by cpuset isolated partitions. Since
> > HK_TYPE_DOMAIN will soon contain both and be subject to runtime
> > modifications, synchronize against housekeeping using the relevant lock.
> > 
> > For full support of cpuset changes, the block subsystem may need to
> > propagate changes to isolated cpumask through the workqueue in the
> > future.
> > 
> > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > ---
> >  block/blk-mq.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index 1978eef95dca..0037af1216f3 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -4257,12 +4257,16 @@ static void blk_mq_map_swqueue(struct request_queue *q)
> >  
> >  		/*
> >  		 * Rule out isolated CPUs from hctx->cpumask to avoid
> > -		 * running block kworker on isolated CPUs
> > +		 * running block kworker on isolated CPUs.
> > +		 * FIXME: cpuset should propagate further changes to isolated CPUs
> > +		 * here.
> >  		 */
> > +		rcu_read_lock();
> >  		for_each_cpu(cpu, hctx->cpumask) {
> >  			if (cpu_is_isolated(cpu))
> >  				cpumask_clear_cpu(cpu, hctx->cpumask);
> >  		}
> > +		rcu_read_unlock();
> 
> Want me to just take this one separately and get it out of your hair?
> Doesn't seem to have any dependencies.

The patch could be applied alone but the rest of the patchset needs it,
otherwise it may dereference freed memory. So I fear it needs to stay
within the lot.

I appreciate the offer though. But an ack would help, even if I must admit
this single patch (which doesn't change current behaviour) leaves a
bitter taste because complete handling of cpuset isolated partition change
will require more work.

Speaking of, is there a way that I missed to define/overwrite the above
hctx->cpumask on runtime?

Thanks.
-- 
Frederic Weisbecker
SUSE Labs