[Bug 101011] Kernel Oops when disconnecting a mounted ext4 usb stick

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=101011

--- Comment #8 from Theodore Tso <tytso@mit.edu> ---
On Fri, Aug 14, 2015 at 11:02:14AM +0000, bugzilla-daemon@bugzilla.kernel.org
wrote:
> https://bugzilla.kernel.org/show_bug.cgi?id=101011
> 
> I can also confirm that this bug is present in latest stable kernel (4.1.5) and
> reverting commit from comment 6 seems to fix it.

Christoph,

I've since gotten two reports from users that reverting your commit:
"08439fec266c3: ext4: remove block_device_ejected" fixes a crash when
a USB stick is yanked from their system.  Looking at the reported
stack dump, it looks like the crash is happening in
account_page_dirtied() when it updates some bdi-specific statistics.

I haven't been paying attention to the recent changes in how bdi gets
torn down after the device gets removed, and in fact finding the
recent changes wasn't obvioius enough after doing a brief search, but
it seems to me that if reverting this patch is making any kind of
differences, then the assertion in the commit description:

    bdi->dev now never goes away, so this function became useless.

it implies that bdi->dev *does* become NULL, and checking for this is
useful.  In any case, I don't see any harm in reverting this commit;
what do you think?

Thanks,

                    - Ted

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