linux-kernel - Re: [PATCH] f2fs: avoid the deadlock case when stopping discard thread

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Date: Thu, 21 Mar 2024 17:29:03 -0700
From: Jaegeuk Kim <jaegeuk@...nel.org>
To: Hillf Danton <hdanton@...a.com>
Cc: linux-kernel@...r.kernel.org,
	Light Hsieh (謝明燈) <Light.Hsieh@...iatek.com>,
	linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [PATCH] f2fs: avoid the deadlock case when stopping discard
 thread

On 03/22, Hillf Danton wrote:
> On Tue, 19 Mar 2024 17:14:42 -0700 Jaegeuk Kim <jaegeuk@...nel.org>
> > f2fs_ioc_shutdown(F2FS_GOING_DOWN_NOSYNC)  issue_discard_thread
> >  - mnt_want_write_file()
> >    - sb_start_write(SB_FREEZE_WRITE)
> 	 __sb_start_write()
> 	   percpu_down_read()
> >                                              - sb_start_intwrite(SB_FREEZE_FS);
> 						   __sb_start_write()
> 						     percpu_down_read()
> 
> Given lock acquirers for read on both sides, wtf deadlock are you fixing?

Damn. I couldn't think _write uses _read sem.

> 
> >  - f2fs_stop_checkpoint(sbi, false,            : waiting
> >     STOP_CP_REASON_SHUTDOWN);
> >  - f2fs_stop_discard_thread(sbi);
> >    - kthread_stop()
> >      : waiting
> > 
> >  - mnt_drop_write_file(filp);
> 
> More important, feel free to add in spin.

I posted this patch before Light reported.

And, in the report, I didn't get this:

f2fs_ioc_shutdown() --> freeze_bdev() --> freeze_super() --> sb_wait_write(sb, SB_FREEZE_FS) --> ... ->percpu_down_write().

because f2fs_ioc_shutdown() calls f2fs_stop_discard_thread() after thaw_bdev()
like this order.

 -> freeze_bdev()
 -> thaw_bdev()
 -> f2fs_stop_discard_thread()

Am I missing something?

> 
> 	Reported-by: "Light Hsieh (謝明燈)" <Light.Hsieh@...iatek.com>