fs: wait for partially frozen filesystems

Jan Kara suggested that when one thread is in the middle of freezing a
filesystem, another thread trying to freeze the same fs but with a
different freeze_holder should wait until the freezer reaches either end
state (UNFROZEN or COMPLETE) instead of returning EBUSY immediately.

Neither caller can do anything sensible with this race other than retry
but they cannot really distinguish EBUSY as in "some other holder of the
same type has the sb already frozen" from "freezing raced with holder of
a different type".

Plumb in the extra code needed to wait for the fs freezer to reach an
end state and try the freeze again.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>

fs/super.c

@@ -1644,6 +1644,24 @@ static void sb_freeze_unlock(struct super_block *sb, int level)
 		percpu_up_write(sb->s_writers.rw_sem + level);
 }
 
+static int wait_for_partially_frozen(struct super_block *sb)
+{
+	int ret = 0;
+
+	do {
+		unsigned short old = sb->s_writers.frozen;
+
+		up_write(&sb->s_umount);
+		ret = wait_var_event_killable(&sb->s_writers.frozen,
+					       sb->s_writers.frozen != old);
+		down_write(&sb->s_umount);
+	} while (ret == 0 &&
+		 sb->s_writers.frozen != SB_UNFROZEN &&
+		 sb->s_writers.frozen != SB_FREEZE_COMPLETE);
+
+	return ret;
+}
+
 /**
  * freeze_super - lock the filesystem and force it into a consistent state
  * @sb: the super to lock
@@ -1695,6 +1713,7 @@ int freeze_super(struct super_block *sb, enum freeze_holder who)
 	atomic_inc(&sb->s_active);
 	down_write(&sb->s_umount);
 
+retry:
 	if (sb->s_writers.frozen == SB_FREEZE_COMPLETE) {
 		if (sb->s_writers.freeze_holders & who) {
 			deactivate_locked_super(sb);
@@ -1713,8 +1732,13 @@ int freeze_super(struct super_block *sb, enum freeze_holder who)
 	}
 
 	if (sb->s_writers.frozen != SB_UNFROZEN) {
-		deactivate_locked_super(sb);
-		return -EBUSY;
+		ret = wait_for_partially_frozen(sb);
+		if (ret) {
+			deactivate_locked_super(sb);
+			return ret;
+		}
+
+		goto retry;
 	}
 
 	if (!(sb->s_flags & SB_BORN)) {
@@ -1726,6 +1750,7 @@ int freeze_super(struct super_block *sb, enum freeze_holder who)
 		/* Nothing to do really... */
 		sb->s_writers.freeze_holders |= who;
 		sb->s_writers.frozen = SB_FREEZE_COMPLETE;
+		wake_up_var(&sb->s_writers.frozen);
 		up_write(&sb->s_umount);
 		return 0;
 	}
@@ -1745,6 +1770,7 @@ int freeze_super(struct super_block *sb, enum freeze_holder who)
 	if (ret) {
 		sb->s_writers.frozen = SB_UNFROZEN;
 		sb_freeze_unlock(sb, SB_FREEZE_PAGEFAULT);
+		wake_up_var(&sb->s_writers.frozen);
 		deactivate_locked_super(sb);
 		return ret;
 	}
@@ -1760,6 +1786,7 @@ int freeze_super(struct super_block *sb, enum freeze_holder who)
 				"VFS:Filesystem freeze failed\n");
 			sb->s_writers.frozen = SB_UNFROZEN;
 			sb_freeze_unlock(sb, SB_FREEZE_FS);
+			wake_up_var(&sb->s_writers.frozen);
 			deactivate_locked_super(sb);
 			return ret;
 		}
@@ -1770,6 +1797,7 @@ int freeze_super(struct super_block *sb, enum freeze_holder who)
 	 */
 	sb->s_writers.freeze_holders |= who;
 	sb->s_writers.frozen = SB_FREEZE_COMPLETE;
+	wake_up_var(&sb->s_writers.frozen);
 	lockdep_sb_freeze_release(sb);
 	up_write(&sb->s_umount);
 	return 0;
@@ -1810,6 +1838,7 @@ static int thaw_super_locked(struct super_block *sb, enum freeze_holder who)
 	if (sb_rdonly(sb)) {
 		sb->s_writers.freeze_holders &= ~who;
 		sb->s_writers.frozen = SB_UNFROZEN;
+		wake_up_var(&sb->s_writers.frozen);
 		goto out;
 	}
 
@@ -1828,6 +1857,7 @@ static int thaw_super_locked(struct super_block *sb, enum freeze_holder who)
 
 	sb->s_writers.freeze_holders &= ~who;
 	sb->s_writers.frozen = SB_UNFROZEN;
+	wake_up_var(&sb->s_writers.frozen);
 	sb_freeze_unlock(sb, SB_FREEZE_FS);
 out:
 	deactivate_locked_super(sb);