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From:   JunChao Sun <sunjunchao2870@gmail.com>
Date:   Thu, 13 Apr 2023 10:19:00 +0800
Message-ID: <CAHB1Nag8uRkBMGv6q2M6C-pSTVxju8im8YRhQwXgEnPVQfscgQ@mail.gmail.com>
Subject: Re: [PATCH] ext4: remove BUG_ON which will be triggered in race scenario
To:     Jan Kara <jack@suse.cz>
Cc:     linux-ext4@vger.kernel.org, tytso@mit.edu, yi.zhang@huawei.com,
        sunjunchao <sunjunchao@yanrongyun.com>
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Jan Kara <jack@suse.cz> =E4=BA=8E2023=E5=B9=B44=E6=9C=8812=E6=97=A5=E5=91=
=A8=E4=B8=89 18:45=E5=86=99=E9=81=93=EF=BC=9A
>
> On Wed 12-04-23 00:47:37, JunChao Sun wrote:
> > From: sunjunchao <sunjunchao@yanrongyun.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_inl=
ine_data()
> >                                                     ->ext4_convert_inli=
ne_data_to_extent()
> >                                                         ->ext4_write_lo=
ck_xattr()
> >                                                             ->ext4_dest=
roy_inline_data_nolock()
> >                                                                 ->ext4_=
clear_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA);
> >                                                         ->ext4_write_un=
lock_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_dat=
a()
> > without holding xattr_sem, and ext4_write_inline_data_end() test it aga=
in 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.

Yes, you are right. The write_begin() and write_end() were protected
by inode->i_rwsem. And then I checked the operations to
EXT4_INODE_INLINE_DATA, but didn't find any race. I didn't hit that, I
was reading code and recognized that there may be a race, but wasn't
aware of the inode->i_rwsem due to my carelessness. Sorry for that.

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