linux-kernel - Re: [PATCH 1/3] cgroup/cpuset: Use static_branch_enable_cpuslocked() on cpusets_insane_config

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Message-ID: <aJSm2sG1G_mk_1P-@jlelli-thinkpadt14gen4.remote.csb>
Date: Thu, 7 Aug 2025 14:15:06 +0100
From: Juri Lelli <juri.lelli@...hat.com>
To: Waiman Long <longman@...hat.com>
Cc: Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>,
	Michal Koutný <mkoutny@...e.com>,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...nel.org>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: Re: [PATCH 1/3] cgroup/cpuset: Use static_branch_enable_cpuslocked()
 on cpusets_insane_config_key

Hi,

On 06/08/25 13:24, Waiman Long wrote:
> The following lockdep splat was observed.
> 
> [  812.359086] ============================================
> [  812.359089] WARNING: possible recursive locking detected
> [  812.359097] --------------------------------------------
> [  812.359100] runtest.sh/30042 is trying to acquire lock:
> [  812.359105] ffffffffa7f27420 (cpu_hotplug_lock){++++}-{0:0}, at: static_key_enable+0xe/0x20
> [  812.359131]
> [  812.359131] but task is already holding lock:
> [  812.359134] ffffffffa7f27420 (cpu_hotplug_lock){++++}-{0:0}, at: cpuset_write_resmask+0x98/0xa70
>      :
> [  812.359267] Call Trace:
> [  812.359272]  <TASK>
> [  812.359367]  cpus_read_lock+0x3c/0xe0
> [  812.359382]  static_key_enable+0xe/0x20
> [  812.359389]  check_insane_mems_config.part.0+0x11/0x30
> [  812.359398]  cpuset_write_resmask+0x9f2/0xa70
> [  812.359411]  cgroup_file_write+0x1c7/0x660
> [  812.359467]  kernfs_fop_write_iter+0x358/0x530
> [  812.359479]  vfs_write+0xabe/0x1250
> [  812.359529]  ksys_write+0xf9/0x1d0
> [  812.359558]  do_syscall_64+0x5f/0xe0
> 
> Since commit d74b27d63a8b ("cgroup/cpuset: Change cpuset_rwsem
> and hotplug lock order"), the ordering of cpu hotplug lock
> and cpuset_mutex had been reversed. That patch correctly
> used the cpuslocked version of the static branch API to enable
> cpusets_pre_enable_key and cpusets_enabled_key, but it didn't do the
> same for cpusets_insane_config_key.
> 
> The cpusets_insane_config_key can be enabled in the
> check_insane_mems_config() which is called from update_nodemask()
> or cpuset_hotplug_update_tasks() with both cpu hotplug lock and
> cpuset_mutex held. Deadlock can happen with a pending hotplug event that
> tries to acquire the cpu hotplug write lock which will block further
> cpus_read_lock() attempt from check_insane_mems_config(). Fix that by
> switching to use static_branch_enable_cpuslocked().
> 
> Fixes: d74b27d63a8b ("cgroup/cpuset: Change cpuset_rwsem and hotplug lock order")
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---

Looks good to me. Thanks for spotting and fixing this.

Reviewed-by: Juri Lelli <juri.lelli@...hat.com>

Best,
Juri