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, 47 insertions, 1 deletions

diff --git a/fs/super.c b/fs/super.c
index 1cb26a3e3df0..19eb70b374bc 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -465,6 +465,48 @@ rescan:
 }
 
 EXPORT_SYMBOL(get_super);
+
+/**
+ * get_active_super - get an active reference to the superblock of a device
+ * @bdev: device to get the superblock for
+ *
+ * Scans the superblock list and finds the superblock of the file system
+ * mounted on the device given.  Returns the superblock with an active
+ * reference and s_umount held exclusively or %NULL if none was found.
+ */
+struct super_block *get_active_super(struct block_device *bdev)
+{
+        struct super_block *sb;
+
+        if (!bdev)
+                return NULL;
+
+        spin_lock(&sb_lock);
+        list_for_each_entry(sb, &super_blocks, s_list) {
+                if (sb->s_bdev != bdev)
+                        continue;
+
+                sb->s_count++;
+                spin_unlock(&sb_lock);
+                down_write(&sb->s_umount);
+                if (sb->s_root) {
+                        spin_lock(&sb_lock);
+                        if (sb->s_count > S_BIAS) {
+                                atomic_inc(&sb->s_active);
+                                sb->s_count--;
+                                spin_unlock(&sb_lock);
+                                return sb;
+                        }
+                        spin_unlock(&sb_lock);
+                }
+                up_write(&sb->s_umount);
+                put_super(sb);
+                yield();
+                spin_lock(&sb_lock);
+        }
+        spin_unlock(&sb_lock);
+        return NULL;
+}
  
 struct super_block * user_get_super(dev_t dev)
 {
@@ -527,11 +569,15 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
 {
         int retval;
         int remount_rw;
-        
+
+        if (sb->s_frozen != SB_UNFROZEN)
+                return -EBUSY;
+
 #ifdef CONFIG_BLOCK
         if (!(flags & MS_RDONLY) && bdev_read_only(sb->s_bdev))
                 return -EACCES;
 #endif
+
         if (flags & MS_RDONLY)
                 acct_auto_close(sb);
         shrink_dcache_sb(sb);