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Message-ID: <ba48eac5-8ef7-251b-11fe-8163bb7a2d54@quicinc.com>
Date:   Wed, 20 Jul 2022 16:36:50 +0530
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Subject: Re: Query regarding deadlock involving cgroup_threadgroup_rwsem and
 cpu_hotplug_lock
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To:     Imran Khan <imran.f.khan@oracle.com>, <tj@kernel.org>,
        <lizefan.x@bytedance.com>, <hannes@cmpxchg.org>,
        <tglx@linutronix.de>, <steven.price@arm.com>,
        <peterz@infradead.org>
CC:     <cgroups@vger.kernel.org>, <linux-kernel@vger.kernel.org>
References: <8245b710-8acb-d8e6-7045-99a5f71dad4e@oracle.com>
 <26d0e4cc-be0e-2c12-6174-dfbb1edb1ed6@oracle.com>
 <bbc01477-231b-3dbb-3e09-9338f5413f06@oracle.com>
From:   Mukesh Ojha <quic_mojha@quicinc.com>
In-Reply-To: <bbc01477-231b-3dbb-3e09-9338f5413f06@oracle.com>
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Hi,

On 7/20/2022 8:57 AM, Imran Khan wrote:
> Hello everyone,
> 
> I am seeing a deadlock between cgroup_threadgroup_rwsem and cpu_hotplug_lock in
> 5.4 kernel.
> 
> Due to some missing drivers I don't have this test setup for latest upstream
> kernel but looking at the code the issue seems to be present in the latest
> kernel as well. If needed I can provide stack traces and other relevant info
> from the vmcore that I have got from 5.4 setup.
> 
> The description of the problem is as follows (I am using 5.19-rc7 as reference
> below):
> 
> __cgroup_procs_write acquires cgroup_threadgroup_rwsem via
> cgroup_procs_write_start and then invokes cgroup_attach_task. Now
> cgroup_attach_task can invoke following call chain:
> 
> cgroup_attach_task --> cgroup_migrate --> cgroup_migrate_execute --> cpuset_attach
> 
> Here cpuset_attach tries to take cpu_hotplug_lock.
> 
> But by this time if some other context
> 
> 1. is already in the middle of cpu hotplug and has acquired cpu_hotplug_lock in
> _cpu_up but
> 2. has not yet reached CPUHP_ONLINE state and
> 3. one of the intermediate hotplug states (in my case CPUHP_AP_ONLINE_DYN ) has
> a callback which involves creation of a thread (or invocation of copy_process
> via some other path) the invoked copy_process will get blocked on
> cgroup_threadgroup_rwsem in following call chain:
> 
>     copy_process --> cgroup_can_fork --> cgroup_css_set_fork -->
> cgroup_threadgroup_change_begin

Similar discussion is at [1], not sure on the conclusion.

[1]
https://lore.kernel.org/lkml/20220705123705.764-1-xuewen.yan@unisoc.com/

-Mukesh

> 
> 
> I am looking for suggestions to fix this deadlock.
> 
> Or if I am missing something in the above analysis and the above mention
> scenario can't happen in latest upstream kernel, then please let me know as that
> would help me in back porting relevant changes to 5.4 kernel because the issue
> definitely exists in 5.4 kernel.
> 
> Thanks,
> -- Imran