block: implement bd_claiming and claiming block

Currently, device claiming for exclusive open is done after low level open - disk->fops->open() - has completed successfully. This means that exclusive open attempts while a device is already exclusively open will fail only after disk->fops->open() is called. cdrom driver issues commands during open() which means that O_EXCL open attempt can unintentionally inject commands to in-progress command stream for burning thus disturbing burning process. In most cases, this doesn't cause problems because the first command to be issued is TUR which most devices can process in the middle of burning. However, depending on how a device replies to TUR during burning, cdrom driver may end up issuing further commands. This can't be resolved trivially by moving bd_claim() before doing actual open() because that means an open attempt which will end up failing could interfere other legit O_EXCL open attempts. ie. unconfirmed open attempts can fail others. This patch resolves the problem by introducing claiming block which is started by bd_start_claiming() and terminated either by bd_claim() or bd_abort_claiming(). bd_claim() from inside a claiming block is guaranteed to succeed and once a claiming block is started, other bd_start_claiming() or bd_claim() attempts block till the current claiming block is terminated. bd_claim() can still be used standalone although now it always synchronizes against claiming blocks, so the existing users will keep working without any change. blkdev_open() and open_bdev_exclusive() are converted to use claiming blocks so that exclusive open attempts from these functions don't interfere with the existing exclusive open. This problem was discovered while investigating bko#15403. https://bugzilla.kernel.org/show_bug.cgi?id=15403 The burning problem itself can be resolved by updating userspace probing tools to always open w/ O_EXCL. Signed-off-by: Tejun Heo <tj@kernel.org> Reported-by: Matthias-Christian Ott <ott@mirix.org> Cc: Kay Sievers <kay.sievers@vrfy.org> Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2010-04-07 18:53:59 +09:00 · 2010-04-07 18:53:59 +09:00 · 6b4517a791
commit 6b4517a791
parent 1a3cbbc5a5
2 changed files with 175 additions and 24 deletions
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@ -693,12 +693,145 @@ static bool bd_may_claim(struct block_device *bdev, struct block_device *whole,
 		return true;	 /* is a partition of an un-held device */
 }

+/**
+ * bd_prepare_to_claim - prepare to claim a block device
+ * @bdev: block device of interest
+ * @whole: the whole device containing @bdev, may equal @bdev
+ * @holder: holder trying to claim @bdev
+ *
+ * Prepare to claim @bdev.  This function fails if @bdev is already
+ * claimed by another holder and waits if another claiming is in
+ * progress.  This function doesn't actually claim.  On successful
+ * return, the caller has ownership of bd_claiming and bd_holder[s].
+ *
+ * CONTEXT:
+ * spin_lock(&bdev_lock).  Might release bdev_lock, sleep and regrab
+ * it multiple times.
+ *
+ * RETURNS:
+ * 0 if @bdev can be claimed, -EBUSY otherwise.
+ */
+static int bd_prepare_to_claim(struct block_device *bdev,
+			       struct block_device *whole, void *holder)
+{
+retry:
+	/* if someone else claimed, fail */
+	if (!bd_may_claim(bdev, whole, holder))
+		return -EBUSY;
+
+	/* if someone else is claiming, wait for it to finish */
+	if (whole->bd_claiming && whole->bd_claiming != holder) {
+		wait_queue_head_t *wq = bit_waitqueue(&whole->bd_claiming, 0);
+		DEFINE_WAIT(wait);
+
+		prepare_to_wait(wq, &wait, TASK_UNINTERRUPTIBLE);
+		spin_unlock(&bdev_lock);
+		schedule();
+		finish_wait(wq, &wait);
+		spin_lock(&bdev_lock);
+		goto retry;
+	}
+
+	/* yay, all mine */
+	return 0;
+}
+
+/**
+ * bd_start_claiming - start claiming a block device
+ * @bdev: block device of interest
+ * @holder: holder trying to claim @bdev
+ *
+ * @bdev is about to be opened exclusively.  Check @bdev can be opened
+ * exclusively and mark that an exclusive open is in progress.  Each
+ * successful call to this function must be matched with a call to
+ * either bd_claim() or bd_abort_claiming().  If this function
+ * succeeds, the matching bd_claim() is guaranteed to succeed.
+ *
+ * CONTEXT:
+ * Might sleep.
+ *
+ * RETURNS:
+ * Pointer to the block device containing @bdev on success, ERR_PTR()
+ * value on failure.
+ */
+static struct block_device *bd_start_claiming(struct block_device *bdev,
+					      void *holder)
+{
+	struct gendisk *disk;
+	struct block_device *whole;
+	int partno, err;
+
+	might_sleep();
+
+	/*
+	 * @bdev might not have been initialized properly yet, look up
+	 * and grab the outer block device the hard way.
+	 */
+	disk = get_gendisk(bdev->bd_dev, &partno);
+	if (!disk)
+		return ERR_PTR(-ENXIO);
+
+	whole = bdget_disk(disk, 0);
+	put_disk(disk);
+	if (!whole)
+		return ERR_PTR(-ENOMEM);
+
+	/* prepare to claim, if successful, mark claiming in progress */
+	spin_lock(&bdev_lock);
+
+	err = bd_prepare_to_claim(bdev, whole, holder);
+	if (err == 0) {
+		whole->bd_claiming = holder;
+		spin_unlock(&bdev_lock);
+		return whole;
+	} else {
+		spin_unlock(&bdev_lock);
+		bdput(whole);
+		return ERR_PTR(err);
+	}
+}
+
+/* releases bdev_lock */
+static void __bd_abort_claiming(struct block_device *whole, void *holder)
+{
+	BUG_ON(whole->bd_claiming != holder);
+	whole->bd_claiming = NULL;
+	wake_up_bit(&whole->bd_claiming, 0);
+
+	spin_unlock(&bdev_lock);
+	bdput(whole);
+}
+
+/**
+ * bd_abort_claiming - abort claiming a block device
+ * @whole: whole block device returned by bd_start_claiming()
+ * @holder: holder trying to claim @bdev
+ *
+ * Abort a claiming block started by bd_start_claiming().  Note that
+ * @whole is not the block device to be claimed but the whole device
+ * returned by bd_start_claiming().
+ *
+ * CONTEXT:
+ * Grabs and releases bdev_lock.
+ */
+static void bd_abort_claiming(struct block_device *whole, void *holder)
+{
+	spin_lock(&bdev_lock);
+	__bd_abort_claiming(whole, holder);		/* releases bdev_lock */
+}
+
 /**
 * bd_claim - claim a block device
 * @bdev: block device to claim
 * @holder: holder trying to claim @bdev
 *
- * Try to claim @bdev.
+ * Try to claim @bdev which must have been opened successfully.  This
+ * function may be called with or without preceding
+ * blk_start_claiming().  In the former case, this function is always
+ * successful and terminates the claiming block.
+ *
+ * CONTEXT:
+ * Might sleep.
 *
 * RETURNS:
 * 0 if successful, -EBUSY if @bdev is already claimed.
@ -706,11 +839,14 @@ static bool bd_may_claim(struct block_device *bdev, struct block_device *whole,
 int bd_claim(struct block_device *bdev, void *holder)
 {
 	struct block_device *whole = bdev->bd_contains;
-	int res = -EBUSY;
+	int res;
+
+	might_sleep();

 	spin_lock(&bdev_lock);

-	if (bd_may_claim(bdev, whole, holder)) {
+	res = bd_prepare_to_claim(bdev, whole, holder);
+	if (res == 0) {
 		/* note that for a whole device bd_holders
 		 * will be incremented twice, and bd_holder will
 		 * be set to bd_claim before being set to holder
@ -719,10 +855,13 @@ int bd_claim(struct block_device *bdev, void *holder)
 		whole->bd_holder = bd_claim;
 		bdev->bd_holders++;
 		bdev->bd_holder = holder;
-		res = 0;
 	}

-	spin_unlock(&bdev_lock);
+	if (whole->bd_claiming)
+		__bd_abort_claiming(whole, holder);	/* releases bdev_lock */
+	else
+		spin_unlock(&bdev_lock);
+
 	return res;
 }
 EXPORT_SYMBOL(bd_claim);
@ -1338,6 +1477,7 @@ EXPORT_SYMBOL(blkdev_get);

 static int blkdev_open(struct inode * inode, struct file * filp)
 {
+	struct block_device *whole = NULL;
 	struct block_device *bdev;
 	int res;

@ -1360,22 +1500,25 @@ static int blkdev_open(struct inode * inode, struct file * filp)
 	if (bdev == NULL)
 		return -ENOMEM;

+	if (filp->f_mode & FMODE_EXCL) {
+		whole = bd_start_claiming(bdev, filp);
+		if (IS_ERR(whole)) {
+			bdput(bdev);
+			return PTR_ERR(whole);
+		}
+	}
+
 	filp->f_mapping = bdev->bd_inode->i_mapping;

 	res = blkdev_get(bdev, filp->f_mode);
-	if (res)
-		return res;

-	if (filp->f_mode & FMODE_EXCL) {
-		res = bd_claim(bdev, filp);
-		if (res)
-			goto out_blkdev_put;
+	if (whole) {
+		if (res == 0)
+			BUG_ON(bd_claim(bdev, filp) != 0);
+		else
+			bd_abort_claiming(whole, filp);
 	}

-	return 0;
-
- out_blkdev_put:
-	blkdev_put(bdev, filp->f_mode);
 	return res;
 }

@ -1586,27 +1729,34 @@ EXPORT_SYMBOL(lookup_bdev);
 */
 struct block_device *open_bdev_exclusive(const char *path, fmode_t mode, void *holder)
 {
-	struct block_device *bdev;
-	int error = 0;
+	struct block_device *bdev, *whole;
+	int error;

 	bdev = lookup_bdev(path);
 	if (IS_ERR(bdev))
 		return bdev;

+	whole = bd_start_claiming(bdev, holder);
+	if (IS_ERR(whole)) {
+		bdput(bdev);
+		return whole;
+	}
+
 	error = blkdev_get(bdev, mode);
 	if (error)
-		return ERR_PTR(error);
+		goto out_abort_claiming;
+
 	error = -EACCES;
 	if ((mode & FMODE_WRITE) && bdev_read_only(bdev))
-		goto blkdev_put;
-	error = bd_claim(bdev, holder);
-	if (error)
-		goto blkdev_put;
+		goto out_blkdev_put;

+	BUG_ON(bd_claim(bdev, holder) != 0);
 	return bdev;
-	
-blkdev_put:
+
+out_blkdev_put:
 	blkdev_put(bdev, mode);
+out_abort_claiming:
+	bd_abort_claiming(whole, holder);
 	return ERR_PTR(error);
 }

--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@ -651,6 +651,7 @@ struct block_device {
 	int			bd_openers;
 	struct mutex		bd_mutex;	/* open/close mutex */
 	struct list_head	bd_inodes;
+	void *			bd_claiming;
 	void *			bd_holder;
 	int			bd_holders;
 #ifdef CONFIG_SYSFS