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Fix a crash when block device is read and block size is changed at the same time

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The kernel may crash when block size is changed and I/O is issued
simultaneously.

Because some subsystems (udev or lvm) may read any block device anytime,
the bug actually puts any code that changes a block device size in
jeopardy.

The crash can be reproduced if you place "msleep(1000)" to
blkdev_get_blocks just before "bh->b_size = max_blocks <<
inode->i_blkbits;".
Then, run "dd if=/dev/ram0 of=/dev/null bs=4k count=1 iflag=direct"
While it is waiting in msleep, run "blockdev --setbsz 2048 /dev/ram0"
You get a BUG.

The direct and non-direct I/O is written with the assumption that block
size does not change. It doesn't seem practical to fix these crashes
one-by-one there may be many crash possibilities when block size changes
at a certain place and it is impossible to find them all and verify the
code.

This patch introduces a new rw-lock bd_block_size_semaphore. The lock is
taken for read during I/O. It is taken for write when changing block
size. Consequently, block size can't be changed while I/O is being
submitted.

For asynchronous I/O, the patch only prevents block size change while
the I/O is being submitted. The block size can change when the I/O is in
progress or when the I/O is being finished. This is acceptable because
there are no accesses to block size when asynchronous I/O is being
finished.

The patch prevents block size changing while the device is mapped with
mmap.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Mikulas Patocka
2012-09-26 07:46:40 +02:00
committed by Jens Axboe
parent 60ea8226cb
commit b87570f5d3
3 changed files with 65 additions and 3 deletions
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4
include/linux/fs.h
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@@ -725,6 +725,8 @@ struct block_device {
int bd_fsfreeze_count;
/* Mutex for freeze */
struct mutex bd_fsfreeze_mutex;
/* A semaphore that prevents I/O while block size is being changed */
struct rw_semaphore bd_block_size_semaphore;
};
/*
@@ -2565,6 +2567,8 @@ extern int generic_segment_checks(const struct iovec *iov,
unsigned long *nr_segs, size_t *count, int access_flags);
/* fs/block_dev.c */
extern ssize_t blkdev_aio_read(struct kiocb *iocb, const struct iovec *iov,
unsigned long nr_segs, loff_t pos);
extern ssize_t blkdev_aio_write(struct kiocb *iocb, const struct iovec *iov,
unsigned long nr_segs, loff_t pos);
extern int blkdev_fsync(struct file *filp, loff_t start, loff_t end,