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Date: Fri, 22 Mar 2024 22:24:32 +0000
From: Jaegeuk Kim <jaegeuk@kernel.org>
To: Light Hsieh =?utf-8?B?KOisneaYjueHiCk=?= <Light.Hsieh@mediatek.com>
Cc: Hillf Danton <hdanton@sina.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-f2fs-devel@lists.sourceforge.net" <linux-f2fs-devel@lists.sourceforge.net>
Subject: Re: =?utf-8?B?5Zue6KaGOiBbUEFUQ0g=?= =?utf-8?Q?=5D?= f2fs: avoid the
 deadlock case when stopping discard thread
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On 03/22, Light Hsieh (謝明燈) wrote:
> I don't see my added log in sb_free_unlock() which will invoke percpu_up_write to release the write semaphore.

May I ask more details whether thaw_super() was called or not?

> 
> 
> ________________________________
> 寄件者: Jaegeuk Kim <jaegeuk@kernel.org>
> 寄件日期: 2024年3月22日 上午 08:29
> 收件者: Hillf Danton <hdanton@sina.com>
> 副本: linux-kernel@vger.kernel.org <linux-kernel@vger.kernel.org>; Light Hsieh (謝明燈) <Light.Hsieh@mediatek.com>; linux-f2fs-devel@lists.sourceforge.net <linux-f2fs-devel@lists.sourceforge.net>
> 主旨: Re: [PATCH] f2fs: avoid the deadlock case when stopping discard thread
> 
> 
> External email : Please do not click links or open attachments until you have verified the sender or the content.
> 
> On 03/22, Hillf Danton wrote:
> > On Tue, 19 Mar 2024 17:14:42 -0700 Jaegeuk Kim <jaegeuk@kernel.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@mediatek.com>
>