linux-ext4 - Re: [PATCH] ext4: remove BUG_ON which will be triggered in race scenario

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Date:   Wed, 12 Apr 2023 12:45:47 +0200
From:   Jan Kara <jack@...e.cz>
To:     JunChao Sun <sunjunchao2870@...il.com>
Cc:     linux-ext4@...r.kernel.org, tytso@....edu, yi.zhang@...wei.com,
        jack@...e.cz, sunjunchao <sunjunchao@...rongyun.com>
Subject: Re: [PATCH] ext4: remove BUG_ON which will be triggered in race
 scenario

On Wed 12-04-23 00:47:37, JunChao Sun wrote:
> From: sunjunchao <sunjunchao@...rongyun.com>
> 
> There is a BUG_ON statement which will be triggered in the
> following scenario, let's remove it.
> 
> thread0                                         thread1
> ext4_write_begin(inode0)
>     ->ext4_try_to_write_inline_data()
>         written some bits successfully
> ext4_write_end(inode0)
>     ->ext4_write_inline_data_end()
>                                             ext4_write_begin(inode0)
>                                                 ->ext4_try_to_write_inline_data()
>                                                     ->ext4_convert_inline_data_to_extent()
>                                                         ->ext4_write_lock_xattr()
>                                                             ->ext4_destroy_inline_data_nolock()
>                                                                 ->ext4_clear_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA);
>                                                         ->ext4_write_unlock_xattr()
>         ->ext4_write_lock_xattr()
>         ->BUG_ON(!ext4_has_inline_data()) will be triggered
> 
> The problematic logic is that ext4_write_end() test ext4_has_inline_data()
> without holding xattr_sem, and ext4_write_inline_data_end() test it again using
> a BUG_ON() with holding xattr_sem.

Were you able to actually hit this? Because inode->i_rwsem should be
protecting us from races like this so I don't think the above described
scenario can happen.

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