linux-kernel - Re: [cgroup/for-6.19 PATCH] cgroup/cpuset: Make callback_lock a raw_spinlock

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Message-ID: <20251112085124.O5dlZ8Og@linutronix.de>
Date: Wed, 12 Nov 2025 09:51:24 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Waiman Long <longman@...hat.com>
Cc: Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>,
	Michal Koutný <mkoutny@...e.com>,
	Clark Williams <clrkwllms@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
	Chen Ridong <chenridong@...wei.com>, Pingfan Liu <piliu@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [cgroup/for-6.19 PATCH] cgroup/cpuset: Make callback_lock a
 raw_spinlock_t

On 2025-11-11 22:57:59 [-0500], Waiman Long wrote:
> The callback_lock is a spinlock_t which is acquired either to read
> a stable set of cpu or node masks or to modify those masks when
> cpuset_mutex is also acquired. Sometime it may need to go up the
> cgroup hierarchy while holding the lock to find the right set of masks
> to use. Assuming that the depth of the cgroup hierarch is finite and
> typically small, the lock hold time should be limited.

We can't assume that, can we?

> Some externally callable cpuset APIs like cpuset_cpus_allowed() and

cpuset_cpus_allowed() has three callers in kernel/sched/ and all use
GFP_KERNEL shortly before invoking the function in question.

> cpuset_mems_allowed() acquires callback_lock with irq disabled to ensure
This I did not find. But I would ask to rework it somehow that we don't
need to use raw_spinlock_t as a hammer that solves it all.

> stable cpuset data. These APIs currently have the restriction that they
> can't be called when a raw spinlock is being held. This is needed to
> work correctly in a PREEMPT_RT kernel. This requires additional code
> changes to work around this limitation. See [1] for a discussion of that.
> 
> Make these external cpuset APIs more useful by changing callback_lock
> to a raw_spinlock_t to remove this limitation so that they can be called
> from within other raw spinlock critical sections if needed.
> 
> [1] https://lore.kernel.org/lkml/20251110014706.8118-1-piliu@redhat.com/
> 
> Signed-off-by: Waiman Long <longman@...hat.com>

Sebastian