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New: potential data race (likely benign) on inode->i_state (reading and writing to different bits)

Bug 205569 [email protected] 1 participant 6 messages Nov 18, 2019 – Nov 19, 2019

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#0 lore raw

https://bugzilla.kernel.org/show_bug.cgi?id=205569

            Bug ID: 205569
           Summary: potential data race (likely benign) on inode->i_state
                    (reading and writing to different bits)
           Product: File System
           Version: 2.5
    Kernel Version: 5.4-rc5
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: ext4
          Assignee: [email protected]
          Reporter: [email protected]
        Regression: No

I am reporting a potential data race (maybe benign) in the ext4 layer on inode->i_state, with reading and writing to the same byte but different bits:
I_DIRTY_PAGES (bit 2) and I_NEW | I_FREEING (bit 3 and 5), observable during the write-back phase.

The function call trace is shown below:

[Thread 1: SYS_rmdir]
__do_sys_rmdir
do_rmdir

    vfs_rmdir
      ext4_rmdir
        ext4_orphan_add
          [READ] WARN_ON_ONCE(!(inode->i_state & (I_NEW | I_FREEING)) &&
                     !inode_is_locked(inode));

[Thread 2: write-back thread]
wb_workfn
wb_do_writeback

    wb_writeback
      writeback_sb_inodes
        __writeback_single_inode
            [WRITE] dirty = inode->i_state & I_DIRTY;

I could confirm that the WRITE may happen before and after the READ operation by controlling the timing of the two threads, i.e., by setting breakpoints before the WRITE statement.

However, I am not very sure about the implication of such a data race (e.g., causing violations of assumptions). I would appreciate if you could help check on this potential bug and advise whether this is a harmful data race or it is intended. Thank you!

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#1 lore raw

[Bug 205569] potential data race (likely benign) on inode->i_state (reading and writing to different bits)

https://bugzilla.kernel.org/show_bug.cgi?id=205569

Theodore Tso ([email protected]) changed:

What |Removed |Added
----------------------------------------------------------------------------
CC| |[email protected]

--- Comment #1 from Theodore Tso ([email protected]) ---
The writeback thread is only applicable for data files. While rmdir() is only
applicable for directories. Also, in both of these function traces, what you
referenced is i_state bits being *read*:

[WRITE] dirty = inode->i_state & I_DIRTY;
^^^^^ not correct!

That being said, there are places in fs/fs-writeback.c where i_state is modified, and there are code paths where ext4_orphan_add() can be called on regular data files --- just not the ones you've listed in this bug.

Can you recheck the call traces and make sure they are correct?

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#2 lore raw

[Bug 205569] potential data race (likely benign) on inode->i_state (reading and writing to different bits)

In reply to [email protected]

https://bugzilla.kernel.org/show_bug.cgi?id=205569

--- Comment #2 from Meng Xu ([email protected]) --- (In reply to Theodore Tso from comment #1)

Quoted 4 lines

The writeback thread is only applicable for data files. While rmdir() is
only applicable for directories. Also, in both of these function traces,
what you referenced is i_state bits being *read*:

Quoted 8 lines

[WRITE] dirty = inode->i_state & I_DIRTY;
^^^^^ not correct!

That being said, there are places in fs/fs-writeback.c where i_state is modified, and there are code paths where ext4_orphan_add() can be called on regular data files --- just not the ones you've listed in this bug.

Can you recheck the call traces and make sure they are correct?

Hi Ted,

My bad, the [WRITE] location is a few lines down the path, inode->i_state &= ~dirty;

Best Regards,
Meng

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#4 lore raw

[Bug 205569] potential data race (likely benign) on inode->i_state (reading and writing to different bits)

In reply to [email protected]

https://bugzilla.kernel.org/show_bug.cgi?id=205569

--- Comment #4 from Meng Xu ([email protected]) --- (In reply to Theodore Tso from comment #3)

Quoted 2 lines

Yes, it's benign. An inode which is I_NEW or I_FREEING will never be in
the writeback.

Many thanks for the confirmation Ted, in the future, I'll post these unsure cases to the mailing list instead of filing a bug report.

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#5 lore raw

[Bug 205569] potential data race (likely benign) on inode->i_state (reading and writing to different bits)

In reply to [email protected]

https://bugzilla.kernel.org/show_bug.cgi?id=205569

Meng Xu ([email protected]) changed:

What |Removed |Added
----------------------------------------------------------------------------

             Status|NEW                         |RESOLVED
         Resolution|---                         |INVALID
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