linux-kernel - Re: [PATCH] workqueue: Clear worker->pool in the worker thread context

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Message-ID: <87zfod54wq.fsf@linux.ibm.com>
Date: Thu, 12 Sep 2024 16:21:25 +0200
From: "Marc Hartmayer" <mhartmay@...ux.ibm.com>
To: Lai Jiangshan <jiangshanlai@...il.com>, linux-kernel@...r.kernel.org
Cc: Lai Jiangshan <jiangshan.ljs@...group.com>, stable@...r.kernel.org,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] workqueue: Clear worker->pool in the worker thread context

On Thu, Sep 12, 2024 at 11:23 AM +0800, Lai Jiangshan <jiangshanlai@...il.com> wrote:
> From: Lai Jiangshan <jiangshan.ljs@...group.com>
>
> Marc Hartmayer reported:
>         [   23.133876] Unable to handle kernel pointer dereference in virtual kernel address space
>         [   23.133950] Failing address: 0000000000000000 TEID: 0000000000000483
>         [   23.133954] Fault in home space mode while using kernel ASCE.
>         [   23.133957] AS:000000001b8f0007 R3:0000000056cf4007 S:0000000056cf3800 P:000000000000003d
>         [   23.134207] Oops: 0004 ilc:2 [#1] SMP
> 	(snip)
>         [   23.134516] Call Trace:
>         [   23.134520]  [<0000024e326caf28>] worker_thread+0x48/0x430
>         [   23.134525] ([<0000024e326caf18>] worker_thread+0x38/0x430)
>         [   23.134528]  [<0000024e326d3a3e>] kthread+0x11e/0x130
>         [   23.134533]  [<0000024e3264b0dc>] __ret_from_fork+0x3c/0x60
>         [   23.134536]  [<0000024e333fb37a>] ret_from_fork+0xa/0x38
>         [   23.134552] Last Breaking-Event-Address:
>         [   23.134553]  [<0000024e333f4c04>] mutex_unlock+0x24/0x30
>         [   23.134562] Kernel panic - not syncing: Fatal exception: panic_on_oops
>
> With debuging and analysis, worker_thread() accesses to the nullified
> worker->pool when the newly created worker is destroyed before being
> waken-up, in which case worker_thread() can see the result detach_worker()
> reseting worker->pool to NULL at the begining.
>
> Move the code "worker->pool = NULL;" out from detach_worker() to fix the
> problem.
>
> worker->pool had been designed to be constant for regular workers and
> changeable for rescuer. To share attaching/detaching code for regular
> and rescuer workers and to avoid worker->pool being accessed inadvertently
> when the worker has been detached, worker->pool is reset to NULL when
> detached no matter the worker is rescuer or not.
>
> To maintain worker->pool being reset after detached, move the code
> "worker->pool = NULL;" in the worker thread context after detached.
>
> It is either be in the regular worker thread context after PF_WQ_WORKER
> is cleared or in rescuer worker thread context with wq_pool_attach_mutex
> held. So it is safe to do so.
>
> Cc: Marc Hartmayer <mhartmay@...ux.ibm.com>
> Link: https://lore.kernel.org/lkml/87wmjj971b.fsf@linux.ibm.com/
> Reported-by: Marc Hartmayer <mhartmay@...ux.ibm.com>
> Fixes: f4b7b53c94af ("workqueue: Detach workers directly in idle_cull_fn()")
> Cc: stable@...r.kernel.org # v6.11+
> Signed-off-by: Lai Jiangshan <jiangshan.ljs@...group.com>
> ---

[…snip…]

Hi, Lai,

so I tested several things:

1. f4b7b53c94af ("workqueue: Detach workers directly in
   idle_cull_fn()") is the commit that introduced the bug
2. with your fix applied I cannot reproduce the bug

Therefore

Tested-by: Marc Hartmayer <mhartmay@...ux.ibm.com>

Thanks for the quick fix!

---
Kind regards / Beste Grüße
   Marc Hartmayer

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