[PATCH 2/7] fsfreeze: emergency thaw will deadlock on s_umount

[Fernando Luis Vázquez Cao <fernando_b1@xxxxxxxxxxxxx>]

The emergency thaw process uses iterate_super() which holds the
sb->s_umount lock in read mode. The current thaw_super() code takes
the sb->s_umount lock in write mode, hence leading to an instant
deadlock.

Pass the emergency state into the thaw_bdev/thaw_super code to avoid
taking the s_umount lock in this case. We are running under the bdev
freeze mutex, so this is still serialised against freeze despite
only having a read lock on the sb->s_umount. Hence it should be safe
to execute in this manner, especially given that emergency thaw is a
rarely executed "get-out-of-jail" feature.

Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
Signed-off-by: Fernando Luis Vazquez Cao <fernando@xxxxxxxxxxxxx>
---

diff -urNp linux-3.5-rc4-orig/fs/block_dev.c linux-3.5-rc4/fs/block_dev.c
--- linux-3.5-rc4-orig/fs/block_dev.c	2012-06-29 14:03:40.732008807 +0900
+++ linux-3.5-rc4/fs/block_dev.c	2012-06-29 14:20:29.480002824 +0900
@@ -305,13 +305,14 @@ struct super_block *freeze_bdev(struct b
 EXPORT_SYMBOL(freeze_bdev);
 
 /**
- * thaw_bdev  -- unlock filesystem
+ * __thaw_bdev  -- unlock filesystem
  * @bdev:	blockdevice to unlock
  * @sb:		associated superblock
+ * @emergency:	emergency thaw
  *
  * Unlocks the filesystem and marks it writeable again after freeze_bdev().
  */
-int thaw_bdev(struct block_device *bdev, struct super_block *sb)
+static int __thaw_bdev(struct block_device *bdev, struct super_block *sb, int emergency)
 {
 	int error = -EINVAL;
 
@@ -327,15 +328,35 @@ int thaw_bdev(struct block_device *bdev,
 	if (!sb)
 		goto out;
 
-	error = thaw_super(sb);
+	if (emergency)
+		error = thaw_super_emergency(sb);
+	else
+		error = thaw_super(sb);
 	if (error)
 		bdev->bd_fsfreeze_count++;
 out:
 	mutex_unlock(&bdev->bd_fsfreeze_mutex);
 	return error;
 }
+
+/**
+ * thaw_bdev  -- unlock filesystem
+ * @bdev:	blockdevice to unlock
+ * @sb:		associated superblock
+ *
+ * Unlocks the filesystem and marks it writeable again after freeze_bdev().
+ */
+int thaw_bdev(struct block_device *bdev, struct super_block *sb)
+{
+	return __thaw_bdev(bdev, sb, 0);
+}
 EXPORT_SYMBOL(thaw_bdev);
 
+int thaw_bdev_emergency(struct block_device *bdev, struct super_block *sb)
+{
+	return __thaw_bdev(bdev, sb, 1);
+}
+
 static int blkdev_writepage(struct page *page, struct writeback_control *wbc)
 {
 	return block_write_full_page(page, blkdev_get_block, wbc);
diff -urNp linux-3.5-rc4-orig/fs/buffer.c linux-3.5-rc4/fs/buffer.c
--- linux-3.5-rc4-orig/fs/buffer.c	2012-06-27 12:00:47.762616000 +0900
+++ linux-3.5-rc4/fs/buffer.c	2012-06-29 14:21:41.948002825 +0900
@@ -514,7 +514,7 @@ repeat:
 static void do_thaw_one(struct super_block *sb, void *unused)
 {
 	char b[BDEVNAME_SIZE];
-	while (sb->s_bdev && !thaw_bdev(sb->s_bdev, sb))
+	while (sb->s_bdev && !thaw_bdev_emergency(sb->s_bdev, sb))
 		printk(KERN_WARNING "Emergency Thaw on %s\n",
 		       bdevname(sb->s_bdev, b));
 }
diff -urNp linux-3.5-rc4-orig/fs/super.c linux-3.5-rc4/fs/super.c
--- linux-3.5-rc4-orig/fs/super.c	2012-05-21 07:29:13.000000000 +0900
+++ linux-3.5-rc4/fs/super.c	2012-06-29 14:49:57.480003483 +0900
@@ -1221,19 +1221,24 @@ int freeze_super(struct super_block *sb)
 EXPORT_SYMBOL(freeze_super);
 
 /**
- * thaw_super -- unlock filesystem
+ * __thaw_super -- unlock filesystem
  * @sb: the super to thaw
+ * @emergency:	emergency thaw
  *
  * Unlocks the filesystem and marks it writeable again after freeze_super().
+ * If we are doing an emergency thaw, we don't need to grab the sb->s_umount
+ * lock as it is already held.
  */
-int thaw_super(struct super_block *sb)
+static int __thaw_super(struct super_block *sb, int emergency)
 {
-	int error;
+	int error = 0;
+
+	if (!emergency)
+		down_write(&sb->s_umount);
 
-	down_write(&sb->s_umount);
 	if (sb->s_frozen == SB_UNFROZEN) {
-		up_write(&sb->s_umount);
-		return -EINVAL;
+		error = -EINVAL;
+		goto out_unlock;
 	}
 
 	if (sb->s_flags & MS_RDONLY)
@@ -1245,8 +1250,7 @@ int thaw_super(struct super_block *sb)
 			printk(KERN_ERR
 				"VFS:Filesystem thaw failed\n");
 			sb->s_frozen = SB_FREEZE_TRANS;
-			up_write(&sb->s_umount);
-			return error;
+			goto out_unlock;
 		}
 	}
 
@@ -1254,8 +1258,44 @@ out:
 	sb->s_frozen = SB_UNFROZEN;
 	smp_wmb();
 	wake_up(&sb->s_wait_unfrozen);
-	deactivate_locked_super(sb);
+
+	/*
+	 * When called from emergency scope, we cannot grab the s_umount lock
+	 * so we cannot deactivate the superblock. This may leave unbalanced
+	 * superblock references which could prevent unmount, but given this is
+	 * an emergency operation....
+	 */
+	if (!emergency)
+		deactivate_locked_super(sb);
 
 	return 0;
+
+out_unlock:
+	if (!emergency)
+		up_write(&sb->s_umount);
+	return error;
+}
+
+/**
+ * thaw_super -- unlock filesystem
+ * @sb: the super to thaw
+ *
+ * Unlocks the filesystem and marks it writeable again after freeze_super().
+ */
+int thaw_super(struct super_block *sb)
+{
+	return __thaw_super(sb, 0);
 }
 EXPORT_SYMBOL(thaw_super);
+
+/**
+ * thaw_super_emergency  -- unlock filesystem
+ * @sb: the super to thaw
+ *
+ * Unlocks the filesystem and marks it writeable again after freeze_super().
+ * This avoids taking the s_umount lock if it is already held.
+ */
+int thaw_super_emergency(struct super_block *sb)
+{
+	return __thaw_super(sb, 1);
+}
diff -urNp linux-3.5-rc4-orig/include/linux/fs.h linux-3.5-rc4/include/linux/fs.h
--- linux-3.5-rc4-orig/include/linux/fs.h	2012-06-27 12:00:48.894616001 +0900
+++ linux-3.5-rc4/include/linux/fs.h	2012-06-29 14:43:51.096010758 +0900
@@ -1941,6 +1941,7 @@ extern int fd_statfs(int, struct kstatfs
 extern int vfs_ustat(dev_t, struct kstatfs *);
 extern int freeze_super(struct super_block *super);
 extern int thaw_super(struct super_block *super);
+extern int thaw_super_emergency(struct super_block *super);
 extern bool our_mnt(struct vfsmount *mnt);
 
 extern int current_umask(void);
@@ -2096,6 +2097,8 @@ extern void kill_bdev(struct block_devic
 extern struct super_block *freeze_bdev(struct block_device *);
 extern void emergency_thaw_all(void);
 extern int thaw_bdev(struct block_device *bdev, struct super_block *sb);
+extern int thaw_bdev_emergency(struct block_device *bdev,
+			       struct super_block *sb);
 extern int fsync_bdev(struct block_device *);
 #else
 static inline void bd_forget(struct inode *inode) {}
@@ -2112,6 +2115,12 @@ static inline int thaw_bdev(struct block
 {
 	return 0;
 }
+
+static inline int thaw_bdev_emergency(struct block_device *bdev,
+				      struct super_block *sb)
+{
+	return 0;
+}
 #endif
 extern int sync_filesystem(struct super_block *);
 extern const struct file_operations def_blk_fops;


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