linux-ext4 - Re: Report 1 in ext4 and journal based on v5.17-rc1

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Date:   Wed, 23 Feb 2022 12:30:12 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Jan Kara <jack@...e.cz>
Cc:     torvalds@...ux-foundation.org, damien.lemoal@...nsource.wdc.com,
        linux-ide@...r.kernel.org, adilger.kernel@...ger.ca,
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Subject: Re: Report 1 in ext4 and journal based on v5.17-rc1

On Tue, Feb 22, 2022 at 09:27:23AM +0100, Jan Kara wrote:
> On Thu 17-02-22 20:10:03, Byungchul Park wrote:
> > [    7.009608] ===================================================
> > [    7.009613] DEPT: Circular dependency has been detected.
> > [    7.009614] 5.17.0-rc1-00014-g8a599299c0cb-dirty #30 Tainted: G        W
> > [    7.009616] ---------------------------------------------------
> > [    7.009617] summary
> > [    7.009618] ---------------------------------------------------
> > [    7.009618] *** DEADLOCK ***
> > [    7.009618]
> > [    7.009619] context A
> > [    7.009619]     [S] (unknown)(&(bit_wait_table + i)->dmap:0)
> > [    7.009621]     [W] down_write(&ei->i_data_sem:0)
> > [    7.009623]     [E] event(&(bit_wait_table + i)->dmap:0)
> > [    7.009624]
> > [    7.009625] context B
> > [    7.009625]     [S] down_read(&ei->i_data_sem:0)
> > [    7.009626]     [W] wait(&(bit_wait_table + i)->dmap:0)
> > [    7.009627]     [E] up_read(&ei->i_data_sem:0)
> > [    7.009628]
> 
> Looking into this I have noticed that Dept here tracks bitlocks (buffer
> locks in particular) but it apparently treats locks on all buffers as one
> locking class so it conflates lock on superblock buffer with a lock on
> extent tree block buffer. These are wastly different locks with different
> locking constraints. So to avoid false positives in filesystems we will
> need to add annotations to differentiate locks on different buffers (based
> on what the block is used for). Similarly how we e.g. annotate i_rwsem for
> different inodes.

Hi Jan Kara,

I just understood why some guys in this space got mad at Dept reports.
I barely got reports from the lock you mentioned with my system -
precisely speaking only one, even though I've been rebooting my system
many times. But another report that someone gave for me showed there
were a lot of reports from the lock.

Your comment and the report are so much helpful. I need to assign
each's own class first for the buffer locks. Thank you very much.

Thanks,
Byungchul

> 
> 								Honza
> -- 
> Jan Kara <jack@...e.com>
> SUSE Labs, CR