freeze_bdev: grab active reference to frozen superblocks

Currently we held s_umount while a filesystem is frozen, despite that we
might return to userspace and unlock it from a different process.  Instead
grab an active reference to keep the file system busy and add an explicit
check for frozen filesystems in remount and reject the remount instead
of blocking on s_umount.

Add a new get_active_super helper to super.c for use by freeze_bdev that
grabs an active reference to a superblock from a given block device.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 72 insertions, 60 deletions

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 22506eb4a58e..9cf4b926f8e4 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -230,43 +230,54 @@ struct super_block *freeze_bdev(struct block_device *bdev)
         int error = 0;
 
         mutex_lock(&bdev->bd_fsfreeze_mutex);
-        if (bdev->bd_fsfreeze_count > 0) {
-                bdev->bd_fsfreeze_count++;
+        if (++bdev->bd_fsfreeze_count > 1) {
+                /*
+                 * We don't even need to grab a reference - the first call
+                 * to freeze_bdev grab an active reference and only the last
+                 * thaw_bdev drops it.
+                 */
                 sb = get_super(bdev);
+                drop_super(sb);
                 mutex_unlock(&bdev->bd_fsfreeze_mutex);
                 return sb;
         }
-        bdev->bd_fsfreeze_count++;
-
-        sb = get_super(bdev);
-        if (sb && !(sb->s_flags & MS_RDONLY)) {
-                sb->s_frozen = SB_FREEZE_WRITE;
-                smp_wmb();
-
-                sync_filesystem(sb);
-
-                sb->s_frozen = SB_FREEZE_TRANS;
-                smp_wmb();
-
-                sync_blockdev(sb->s_bdev);
-
-                if (sb->s_op->freeze_fs) {
-                        error = sb->s_op->freeze_fs(sb);
-                        if (error) {
-                                printk(KERN_ERR
-                                        "VFS:Filesystem freeze failed\n");
-                                sb->s_frozen = SB_UNFROZEN;
-                                drop_super(sb);
-                                bdev->bd_fsfreeze_count--;
-                                mutex_unlock(&bdev->bd_fsfreeze_mutex);
-                                return ERR_PTR(error);
-                        }
+
+        sb = get_active_super(bdev);
+        if (!sb)
+                goto out;
+        if (sb->s_flags & MS_RDONLY) {
+                deactivate_locked_super(sb);
+                mutex_unlock(&bdev->bd_fsfreeze_mutex);
+                return sb;
+        }
+
+        sb->s_frozen = SB_FREEZE_WRITE;
+        smp_wmb();
+
+        sync_filesystem(sb);
+
+        sb->s_frozen = SB_FREEZE_TRANS;
+        smp_wmb();
+
+        sync_blockdev(sb->s_bdev);
+
+        if (sb->s_op->freeze_fs) {
+                error = sb->s_op->freeze_fs(sb);
+                if (error) {
+                        printk(KERN_ERR
+                                "VFS:Filesystem freeze failed\n");
+                        sb->s_frozen = SB_UNFROZEN;
+                        deactivate_locked_super(sb);
+                        bdev->bd_fsfreeze_count--;
+                        mutex_unlock(&bdev->bd_fsfreeze_mutex);
+                        return ERR_PTR(error);
                 }
         }
+        up_write(&sb->s_umount);
 
+ out:
         sync_blockdev(bdev);
         mutex_unlock(&bdev->bd_fsfreeze_mutex);
-
         return sb;      /* thaw_bdev releases s->s_umount */
 }
 EXPORT_SYMBOL(freeze_bdev);
@@ -280,43 +291,44 @@ EXPORT_SYMBOL(freeze_bdev);
  */
 int thaw_bdev(struct block_device *bdev, struct super_block *sb)
 {
-        int error = 0;
+        int error = -EINVAL;
 
         mutex_lock(&bdev->bd_fsfreeze_mutex);
-        if (!bdev->bd_fsfreeze_count) {
-                mutex_unlock(&bdev->bd_fsfreeze_mutex);
-                return -EINVAL;
-        }
-
-        bdev->bd_fsfreeze_count--;
-        if (bdev->bd_fsfreeze_count > 0) {
-                if (sb)
-                        drop_super(sb);
-                mutex_unlock(&bdev->bd_fsfreeze_mutex);
-                return 0;
-        }
-
-        if (sb) {
-                BUG_ON(sb->s_bdev != bdev);
-                if (!(sb->s_flags & MS_RDONLY)) {
-                        if (sb->s_op->unfreeze_fs) {
-                                error = sb->s_op->unfreeze_fs(sb);
-                                if (error) {
-                                        printk(KERN_ERR
-                                                "VFS:Filesystem thaw failed\n");
-                                        sb->s_frozen = SB_FREEZE_TRANS;
-                                        bdev->bd_fsfreeze_count++;
-                                        mutex_unlock(&bdev->bd_fsfreeze_mutex);
-                                        return error;
-                                }
-                        }
-                        sb->s_frozen = SB_UNFROZEN;
-                        smp_wmb();
-                        wake_up(&sb->s_wait_unfrozen);
+        if (!bdev->bd_fsfreeze_count)
+                goto out_unlock;
+
+        error = 0;
+        if (--bdev->bd_fsfreeze_count > 0)
+                goto out_unlock;
+
+        if (!sb)
+                goto out_unlock;
+
+        BUG_ON(sb->s_bdev != bdev);
+        down_write(&sb->s_umount);
+        if (sb->s_flags & MS_RDONLY)
+                goto out_deactivate;
+
+        if (sb->s_op->unfreeze_fs) {
+                error = sb->s_op->unfreeze_fs(sb);
+                if (error) {
+                        printk(KERN_ERR
+                                "VFS:Filesystem thaw failed\n");
+                        sb->s_frozen = SB_FREEZE_TRANS;
+                        bdev->bd_fsfreeze_count++;
+                        mutex_unlock(&bdev->bd_fsfreeze_mutex);
+                        return error;
                 }
-                drop_super(sb);
         }
 
+        sb->s_frozen = SB_UNFROZEN;
+        smp_wmb();
+        wake_up(&sb->s_wait_unfrozen);
+
+out_deactivate:
+        if (sb)
+                deactivate_locked_super(sb);
+out_unlock:
         mutex_unlock(&bdev->bd_fsfreeze_mutex);
         return 0;
 }