[PATCH 2/3] fs: wait for partially frozen filesystems

["Darrick J. Wong" <djwong@xxxxxxxxxx>]

From: Darrick J. Wong <djwong@xxxxxxxxxx>

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.

Plumb in the extra coded needed to wait for the fs freezer to reach an
end state and try the freeze again.

Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx>
---
 fs/super.c |   27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)


diff --git a/fs/super.c b/fs/super.c
index 36adccecc828..151e0eeff2c2 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1647,6 +1647,15 @@ static int freeze_frozen_super(struct super_block *sb, enum freeze_holder who)
 	return 0;
 }
 
+static void wait_for_partially_frozen(struct super_block *sb)
+{
+	up_write(&sb->s_umount);
+	wait_var_event(&sb->s_writers.frozen,
+			sb->s_writers.frozen == SB_UNFROZEN ||
+			sb->s_writers.frozen == SB_FREEZE_COMPLETE);
+	down_write(&sb->s_umount);
+}
+
 /**
  * freeze_super - lock the filesystem and force it into a consistent state
  * @sb: the super to lock
@@ -1690,11 +1699,13 @@ static int freeze_frozen_super(struct super_block *sb, enum freeze_holder who)
  */
 int freeze_super(struct super_block *sb, enum freeze_holder who)
 {
+	bool try_again = true;
 	int ret;
 
 	atomic_inc(&sb->s_active);
 	down_write(&sb->s_umount);
 
+retry:
 	if (sb->s_writers.frozen == SB_FREEZE_COMPLETE) {
 		ret = freeze_frozen_super(sb, who);
 		deactivate_locked_super(sb);
@@ -1702,8 +1713,14 @@ int freeze_super(struct super_block *sb, enum freeze_holder who)
 	}
 
 	if (sb->s_writers.frozen != SB_UNFROZEN) {
-		deactivate_locked_super(sb);
-		return -EBUSY;
+		if (!try_again) {
+			deactivate_locked_super(sb);
+			return -EBUSY;
+		}
+
+		wait_for_partially_frozen(sb);
+		try_again = false;
+		goto retry;
 	}
 
 	if (!(sb->s_flags & SB_BORN)) {
@@ -1716,6 +1733,7 @@ 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;
 	}
@@ -1736,6 +1754,7 @@ int freeze_super(struct super_block *sb, enum freeze_holder who)
 		sb->s_writers.frozen = SB_UNFROZEN;
 		sb_freeze_unlock(sb, SB_FREEZE_PAGEFAULT);
 		wake_up(&sb->s_writers.wait_unfrozen);
+		wake_up_var(&sb->s_writers.frozen);
 		deactivate_locked_super(sb);
 		return ret;
 	}
@@ -1752,6 +1771,7 @@ int freeze_super(struct super_block *sb, enum freeze_holder who)
 			sb->s_writers.frozen = SB_UNFROZEN;
 			sb_freeze_unlock(sb, SB_FREEZE_FS);
 			wake_up(&sb->s_writers.wait_unfrozen);
+			wake_up_var(&sb->s_writers.frozen);
 			deactivate_locked_super(sb);
 			return ret;
 		}
@@ -1762,6 +1782,7 @@ 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 +1831,7 @@ 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 +1850,7 @@ 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:
 	wake_up(&sb->s_writers.wait_unfrozen);