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    SubjectRe: [PATCH] fs: Fix mod_timer crash when removing USB sticks
    On Thu, Jan 12, 2012 at 01:57:11PM -0800, Paul Taysom wrote:
    > A USB stick with a ext file system on it, would occasionally crash
    > when the stick was pulled.
    >
    > The problem was a timer was being set on the Backing Device Interface,
    > bdi, after the USB device had been removed and the bdi had been
    > unregistered. The bdi would then be later reinitialized by zeroing
    > the timer without removing from the timer from the timer queue.
    > This would eventually result in a kernel crash (NULL ptr dereference).
    >
    > When the bdi is unregistered, the dev field is set to NULL. This
    > indication is used by bdi_unregister to only unregister the device
    > once.
    >
    > Fix: When the backing device is invalidated, the mapping backing_dev_info
    > should be redirected to the default_backing_dev_info.
    >
    > Created 3 USB sticks with ext2, ext4 and one with both apple and DOS
    > file systems on it. Inserted and removed USB sticks many times in random
    > order. With out the bug fix, the kernel would soon crash. With the fix,
    > it did not. Ran on both stumpy and amd64-generic.
    >
    > Signed-off-by: Paul Taysom <taysom@chromium.org>
    > Cc: Mandeep Baines <msb@chromium.org>
    > Cc: Greg KH <greg@kroah.com>
    > Cc: Jens Axboe <axboe@kernel.dk>
    > Cc: Theodore Tso <tytso@google.com>
    > Cc: Andrew Morton <akpm@google.com>
    > Cc: <linux-usb@vger.kernel.org>
    > Cc: <linux-kernel@vger.kernel.org>
    > Cc: Alexander Viro <viro@zeniv.linux.org.uk>
    > Cc: <linux-fsdevel@vger.kernel.org>
    > Cc: <stable@kernel.org>
    > ---
    > fs/block_dev.c | 1 +
    > 1 files changed, 1 insertions(+), 0 deletions(-)
    >
    > diff --git a/fs/block_dev.c b/fs/block_dev.c
    > index afe74dd..322cd05 100644
    > --- a/fs/block_dev.c
    > +++ b/fs/block_dev.c
    > @@ -110,6 +110,7 @@ void invalidate_bdev(struct block_device *bdev)
    > * But, for the strange corners, lets be cautious
    > */
    > cleancache_flush_inode(mapping);
    > + mapping->backing_dev_info = &default_backing_dev_info;
    > }
    > EXPORT_SYMBOL(invalidate_bdev);

    What ever happened to this patch? Is it still needed? Can you still
    reproduce the problem on Linus's tree and older kernels?

    thanks,

    greg k-h


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