fix up lock order reversal in writeback

Commit Message

Nick Piggin Nov. 16, 2010, 11 a.m. UTC

None

Patch

Index: linux-2.6/fs/fs-writeback.c
===================================================================
--- linux-2.6.orig/fs/fs-writeback.c	2010-11-16 21:44:32.000000000 +1100
+++ linux-2.6/fs/fs-writeback.c	2010-11-16 21:49:37.000000000 +1100
@@ -1125,16 +1125,20 @@  EXPORT_SYMBOL(writeback_inodes_sb);
  *
  * Invoke writeback_inodes_sb if no writeback is currently underway.
  * Returns 1 if writeback was started, 0 if not.
+ *
+ * May be called inside i_lock. May not start writeback if locks cannot
+ * be acquired.
  */
 int writeback_inodes_sb_if_idle(struct super_block *sb)
 {
 	if (!writeback_in_progress(sb->s_bdi)) {
-		down_read(&sb->s_umount);
-		writeback_inodes_sb(sb);
-		up_read(&sb->s_umount);
-		return 1;
-	} else
-		return 0;
+		if (down_read_trylock(&sb->s_umount)) {
+			writeback_inodes_sb(sb);
+			up_read(&sb->s_umount);
+			return 1;
+		}
+	}
+	return 0;
 }
 EXPORT_SYMBOL(writeback_inodes_sb_if_idle);
 
@@ -1145,17 +1149,21 @@  EXPORT_SYMBOL(writeback_inodes_sb_if_idl
  *
  * Invoke writeback_inodes_sb if no writeback is currently underway.
  * Returns 1 if writeback was started, 0 if not.
+ *
+ * May be called inside i_lock. May not start writeback if locks cannot
+ * be acquired.
  */
 int writeback_inodes_sb_nr_if_idle(struct super_block *sb,
 				   unsigned long nr)
 {
 	if (!writeback_in_progress(sb->s_bdi)) {
-		down_read(&sb->s_umount);
-		writeback_inodes_sb_nr(sb, nr);
-		up_read(&sb->s_umount);
-		return 1;
-	} else
-		return 0;
+		if (down_read_trylock(&sb->s_umount)) {
+			writeback_inodes_sb_nr(sb, nr);
+			up_read(&sb->s_umount);
+			return 1;
+		}
+	}
+	return 0;
 }
 EXPORT_SYMBOL(writeback_inodes_sb_nr_if_idle);