linux-kernel - Re: [PATCH v3 1/1] cgroup: fix deadlock caused by cgroup_mutex and cpu_hotplug

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Message-ID: <ZuC3femqBNufgX1D@slm.duckdns.org>
Date: Tue, 10 Sep 2024 11:17:49 -1000
From: Tejun Heo <tj@...nel.org>
To: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: Chen Ridong <chenridong@...weicloud.com>,
	Michal Koutný <mkoutny@...e.com>,
	Chen Ridong <chenridong@...wei.com>, martin.lau@...ux.dev,
	ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
	eddyz87@...il.com, song@...nel.org, yonghong.song@...ux.dev,
	john.fastabend@...il.com, kpsingh@...nel.org, sdf@...gle.com,
	haoluo@...gle.com, jolsa@...nel.org, lizefan.x@...edance.com,
	hannes@...xchg.org, bpf@...r.kernel.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/1] cgroup: fix deadlock caused by cgroup_mutex and
 cpu_hotplug_lock

On Tue, Sep 10, 2024 at 09:02:59PM +0000, Roman Gushchin wrote:
...
> > > By that reasoning any holder of cgroup_mutex on system_wq makes system
> > > susceptible to a deadlock (in presence of cpu_hotplug_lock waiting
> > > writers + cpuset operations). And the two work items must meet in same
> > > worker's processing hence probability is low (zero?) with less than
> > > WQ_DFL_ACTIVE items.
> 
> Right, I'm on the same page. Should we document then somewhere that
> the cgroup mutex can't be locked from a system wq context?
> 
> I think thus will also make the Fixes tag more meaningful.

I think that's completely fine. What's not fine is saturating system_wq.
Anything which creates a large number of concurrent work items should be
using its own workqueue. If anything, workqueue needs to add a warning for
saturation conditions and who are the offenders.

Thanks.

-- 
tejun