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Date:   Fri, 5 Mar 2021 11:10:05 +0100
From:   Jan Kara <jack@suse.cz>
To:     "zhangyi (F)" <yi.zhang@huawei.com>
Cc:     Jan Kara <jack@suse.cz>, linux-fsdevel@vger.kernel.org,
        linux-ext4@vger.kernel.org, tytso@mit.edu, viro@zeniv.linux.org.uk,
        linfeilong@huawei.com, Ye Bin <yebin10@huawei.com>
Subject: Re: [PATCH] block_dump: don't put the last refcount when marking
 inode dirty
Message-ID: <20210305101005.GA14142@quack2.suse.cz>
References: <20210226103103.3048803-1-yi.zhang@huawei.com>
 <20210301112102.GD25026@quack2.suse.cz>
 <5f72dc70-9fb0-0d3b-dc31-f60d35929991@huawei.com>
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On Thu 04-03-21 21:37:42, zhangyi (F) wrote:
> On 2021/3/1 19:21, Jan Kara wrote:
> > On Fri 26-02-21 18:31:03, zhangyi (F) wrote:
> >> There is an AA deadlock problem when using block_dump on ext4 file
> >> system with data=journal mode.
> >>
> >>   watchdog: BUG: soft lockup - CPU#19 stuck for 22s! [jbd2/pmem0-8:1002]
> >>   CPU: 19 PID: 1002 Comm: jbd2/pmem0-8
> >>   RIP: 0010:queued_spin_lock_slowpath+0x60/0x3b0
> >>   ...
> >>   Call Trace:
> >>    _raw_spin_lock+0x57/0x70
> >>    jbd2_journal_invalidatepage+0x166/0x680
> >>    __ext4_journalled_invalidatepage+0x8c/0x120
> >>    ext4_journalled_invalidatepage+0x12/0x40
> >>    truncate_cleanup_page+0x10e/0x1c0
> >>    truncate_inode_pages_range+0x2c8/0xec0
> >>    truncate_inode_pages_final+0x41/0x90
> >>    ext4_evict_inode+0x254/0xac0
> >>    evict+0x11c/0x2f0
> >>    iput+0x20e/0x3a0
> >>    dentry_unlink_inode+0x1bf/0x1d0
> >>    __dentry_kill+0x14c/0x2c0
> >>    dput+0x2bc/0x630
> >>    block_dump___mark_inode_dirty.cold+0x5c/0x111
> >>    __mark_inode_dirty+0x678/0x6b0
> >>    mark_buffer_dirty+0x16e/0x1d0
> >>    __jbd2_journal_temp_unlink_buffer+0x127/0x1f0
> >>    __jbd2_journal_unfile_buffer+0x24/0x80
> >>    __jbd2_journal_refile_buffer+0x12f/0x1b0
> >>    jbd2_journal_commit_transaction+0x244b/0x3030
> >>
> >> The problem is a race between jbd2 committing data buffer and user
> >> unlink the file concurrently. The jbd2 will get jh->b_state_lock and
> >> redirty the inode's data buffer and inode itself. If block_dump is
> >> enabled, it will try to find inode's dentry and invoke the last dput()
> >> after the inode was unlinked. Then the evict procedure will unmap
> >> buffer and get jh->b_state_lock again in journal_unmap_buffer(), and
> >> finally lead to deadlock. It works fine if block_dump is not enabled
> >> because the last evict procedure is not invoked in jbd2 progress and
> >> the jh->b_state_lock will also prevent inode use after free.
> >>
> >> jbd2                                xxx
> >>                                     vfs_unlink
> >>                                      ext4_unlink
> >> jbd2_journal_commit_transaction
> >> **get jh->b_state_lock**
> >> jbd2_journal_refile_buffer
> >>  mark_buffer_dirty
> >>   __mark_inode_dirty
> >>    block_dump___mark_inode_dirty
> >>     d_find_alias
> >>                                      d_delete
> >>                                       unhash
> >>     dput  //put the last refcount
> >>      evict
> >>       journal_unmap_buffer
> >>        **get jh->b_state_lock again**
> >>
> >> In most cases of where invoking mark_inode_dirty() will get inode's
> >> refcount and the last iput may not happen, but it's not safe. After
> >> checking the block_dump code, it only want to dump the file name of the
> >> dirty inode, so there is no need to get and put denrty, and dump an
> >> unhashed dentry is also fine. This patch remove the dget() && dput(),
> >> print the dentry name directly.
> >>
> >> Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
> >> Signed-off-by: yebin (H) <yebin10@huawei.com>
> > 
> > Hrm, ok. Honestly, I wanted to just delete that code for a long time. IMO
> > tracepoints (and we have one in __mark_inode_dirty) are much more useful
> > for tracing anyway. This code exists only because it was there much before
> > tracepoints existed... Do you have a strong reason why are you using
> > block_dump instead of tracepoint trace_writeback_mark_inode_dirty() for
> > your monitoring?
> > 
> 
> Hi, Jan. We just do some stress tests and find this issue, I'm not sure who
> are still using this old debug interface and gather it may need time. Could
> we firstly fix this issue, and then delete this code if no opposed?

I'd do it the other way around :) Delete the code and only fix it if
someone complains that the feature is still used and so we should not
delete it. Will you send a patch or should I do it?

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