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    SubjectRe: [PATCH] fs: Fix mod_timer crash when removing USB sticks
    On Fri, 16 Mar 2012, Theodore Tso wrote:

    > I thought another fix at the USB layer also went in that attempted to
    > fix this problem for 3.2, and so with two separate band-aid patches, I
    > think we had thought the problem had been addressed.

    I can't recall any USB fix like that. The only thing I remember is
    your change to ext4 (leaving the problem still present in ext3).

    > The real problem is that all of the patches which I've seen to date
    > are band-aids, in that we aren't properly sending a "device as
    > disappeared" notification to the file system layer, but instead we are
    > trying to keep enough of the pointers valid (while also freeing other
    > data structures), such that the file system can blindly write into a
    > partially dismantled block device, and hopefully not oops.

    That's not a band-aid approach; it's the way reference counting is
    _intended_ to work. The whole idea of refcounting is that instead of
    synchronizing every single operation, you keep data structures around
    so long as anyone might be using them.

    > Some have argued that my suggested approach of having an explicit
    > super_ops revoke() function, which tells the file system that the
    > block device is gone, etc., isn't necessary because this can be solved
    > in userspace somehow. Personally I think that's nuts, since we'll
    > continue to play whack-a-mole, but I haven't had time to work up
    > patches addressing this --- since this is really only a problem for
    > naive users who pull USB sticks without unmounting them first (and so
    > it never happens to me :-), and I've got a lot of other fish to
    > try.....

    I suspect Paul's patch is the right thing to do. It might even make
    the ext4 fix unnecessary, although I don't understand the details well
    enough to verify it. Maybe Paul can check -- the commit I'm referring
    to is 7c2e70879fc0949b4220ee61b7c4553f6976a94d (ext4: add ext4-specific
    kludge to avoid an oops after the disk disappears).

    Alan Stern

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