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Commit 260fa034 authored by NeilBrown's avatar NeilBrown
Browse files

md: avoid deadlock when dirty buffers during md_stop. 



When the last process closes /dev/mdX sync_blockdev will be called so
that all buffers get flushed.
So if it is then opened for the STOP_ARRAY ioctl to be sent there will
be nothing to flush.

However if we open /dev/mdX in order to send the STOP_ARRAY ioctl just
moments before some other process which was writing closes their file
descriptor, then there won't be a 'last close' and the buffers might
not get flushed.

So do_md_stop() calls sync_blockdev().  However at this point it is
holding ->reconfig_mutex.  So if the array is currently 'clean' then
the writes from sync_blockdev() will not complete until the array
can be marked dirty and that won't happen until some other thread
can get ->reconfig_mutex.  So we deadlock.

We need to move the sync_blockdev() call to before we take
->reconfig_mutex.
However then some other thread could open /dev/mdX and write to it
after we call sync_blockdev() and before we actually stop the array.
This can leave dirty data in the page cache which is awkward.

So introduce new flag MD_STILL_CLOSED.  Set it before calling
sync_blockdev(), clear it if anyone does open the file, and abort the
STOP_ARRAY attempt if it gets set before we lock against further
opens.

It is still possible to get problems if you open /dev/mdX, write to
it, then issue the STOP_ARRAY ioctl.  Just don't do that.

Signed-off-by: default avatarNeilBrown <neilb@suse.de>
parent 7a0a5355

drivers/md/md.c

+30 −9
Original line number Diff line number Diff line
@@ -5337,8 +5337,14 @@ static int md_set_readonly(struct mddev *mddev, struct block_device *bdev) 
		err = -EBUSY;

		goto out;

	}

	if (bdev)

		sync_blockdev(bdev);

	if (bdev && !test_bit(MD_STILL_CLOSED, &mddev->flags)) {

		/* Someone opened the device since we flushed it

		 * so page cache could be dirty and it is too late

		 * to flush.  So abort

		 */

		mutex_unlock(&mddev->open_mutex);

		return -EBUSY;

	}

	if (mddev->pers) {

		__md_stop_writes(mddev);
@@ -5373,14 +5379,14 @@ static int do_md_stop(struct mddev * mddev, int mode, 
		mutex_unlock(&mddev->open_mutex);

		return -EBUSY;

	}

	if (bdev)

		/* It is possible IO was issued on some other

		 * open file which was closed before we took ->open_mutex.

		 * As that was not the last close __blkdev_put will not

		 * have called sync_blockdev, so we must.

	if (bdev && !test_bit(MD_STILL_CLOSED, &mddev->flags)) {

		/* Someone opened the device since we flushed it

		 * so page cache could be dirty and it is too late

		 * to flush.  So abort

		 */

		sync_blockdev(bdev);



		mutex_unlock(&mddev->open_mutex);

		return -EBUSY;

	}

	if (mddev->pers) {

		if (mddev->ro)

			set_disk_ro(disk, 0);
@@ -6417,6 +6423,20 @@ static int md_ioctl(struct block_device *bdev, fmode_t mode, 
						 !test_bit(MD_RECOVERY_NEEDED,

							   &mddev->flags),

						 msecs_to_jiffies(5000));

	if (cmd == STOP_ARRAY || cmd == STOP_ARRAY_RO) {

		/* Need to flush page cache, and ensure no-one else opens

		 * and writes

		 */

		mutex_lock(&mddev->open_mutex);

		if (atomic_read(&mddev->openers) > 1) {

			mutex_unlock(&mddev->open_mutex);

			err = -EBUSY;

			goto abort;

		}

		set_bit(MD_STILL_CLOSED, &mddev->flags);

		mutex_unlock(&mddev->open_mutex);

		sync_blockdev(bdev);

	}

	err = mddev_lock(mddev);

	if (err) {

		printk(KERN_INFO
@@ -6670,6 +6690,7 @@ static int md_open(struct block_device *bdev, fmode_t mode) 


	err = 0;

	atomic_inc(&mddev->openers);

	clear_bit(MD_STILL_CLOSED, &mddev->flags);

	mutex_unlock(&mddev->open_mutex);



	check_disk_change(bdev);

drivers/md/md.h

+3 −0
Original line number Diff line number Diff line
@@ -211,6 +211,9 @@ struct mddev { 
#define MD_CHANGE_PENDING 2	/* switch from 'clean' to 'active' in progress */

#define MD_UPDATE_SB_FLAGS (1 | 2 | 4)	/* If these are set, md_update_sb needed */

#define MD_ARRAY_FIRST_USE 3    /* First use of array, needs initialization */

#define MD_STILL_CLOSED	4	/* If set, then array has not been opened since

				 * md_ioctl checked on it.

				 */



	int				suspended;

	atomic_t			active_io;