linux-ext4 - Re: [REPORT] syscall reboot + umh + firmware fallback

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Date:   Wed, 11 May 2022 23:15:09 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Byungchul Park <byungchul.park@....com>
Cc:     torvalds@...ux-foundation.org, holt@....com, mcgrof@...nel.org,
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Subject: Re: [REPORT] syscall reboot + umh + firmware fallback

Hello,

Just took a look out of curiosity.

On Thu, May 12, 2022 at 02:25:57PM +0900, Byungchul Park wrote:
> PROCESS A	PROCESS B	WORKER C
> 
> __do_sys_reboot()
> 		__do_sys_reboot()
>  mutex_lock(&system_transition_mutex)
>  ...		 mutex_lock(&system_transition_mutex) <- stuck
> 		 ...
> 				request_firmware_work_func()
> 				 _request_firmware()
> 				  firmware_fallback_sysfs()
> 				   usermodehelper_read_lock_wait()
> 				    down_read(&umhelper_sem)
> 				   ...
> 				   fw_load_sysfs_fallback()
> 				    fw_sysfs_wait_timeout()
> 				     wait_for_completion_killable_timeout(&fw_st->completion) <- stuck
>  kernel_halt()
>   __usermodehelper_disable()
>    down_write(&umhelper_sem) <- stuck
> 
> --------------------------------------------------------
> All the 3 contexts are stuck at this point.
> --------------------------------------------------------
> 
> PROCESS A	PROCESS B	WORKER C
> 
>    ...
>    up_write(&umhelper_sem)
>  ...
>  mutex_unlock(&system_transition_mutex) <- cannot wake up B
> 
> 		 ...
> 		 kernel_halt()
> 		  notifier_call_chain()
> 		   hw_shutdown_notify()
> 		    kill_pending_fw_fallback_reqs()
> 		     __fw_load_abort()
> 		      complete_all(&fw_st->completion) <- cannot wake up C
> 
> 				   ...
> 				   usermodeheler_read_unlock()
> 				    up_read(&umhelper_sem) <- cannot wake up A

I'm not sure I'm reading it correctly but it looks like "process B" column
is superflous given that it's waiting on the same lock to do the same thing
that A is already doing (besides, you can't really halt the machine twice).
What it's reporting seems to be ABBA deadlock between A waiting on
umhelper_sem and C waiting on fw_st->completion. The report seems spurious:

1. wait_for_completion_killable_timeout() doesn't need someone to wake it up
   to make forward progress because it will unstick itself after timeout
   expires.

2. complete_all() from __fw_load_abort() isn't the only source of wakeup.
   The fw loader can be, and mainly should be, woken up by firmware loading
   actually completing instead of being aborted.

I guess the reason why B shows up there is because the operation order is
such that just between A and C, the complete_all() takes place before
__usermodehlper_disable(), so the whole thing kinda doesn't make sense as
you can't block a past operation by a future one. Inserting process B
introduces the reverse ordering.

Thanks.

-- 
tejun