path: root/include
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authorMikulas Patocka <mpatocka@redhat.com>2012-09-26 07:46:40 +0200
committerJens Axboe <axboe@kernel.dk>2012-09-26 07:46:40 +0200
commitb87570f5d349661814b262dd5fc40787700f80d6 (patch)
treed06ef6c95ed114e19c864ebe0240c788dd75e85c /include
parent60ea8226cbd5c8301f9a39edc574ddabcb8150e0 (diff)
Fix a crash when block device is read and block size is changed at the same time
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>
Diffstat (limited to 'include')
1 files changed, 4 insertions, 0 deletions
diff --git a/include/linux/fs.h b/include/linux/fs.h
index bd6f6e7ca48..e60bbd0225d 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -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,