[PATCH] O(1) sb list traversing on syncs

This patch removes O(n^2) super block loops in sync_inodes(),
sync_filesystems() etc.  in favour of using __put_super_and_need_restart()
which I introduced earlier.  We faced a noticably long freezes on sb
syncing when there are thousands of super blocks in the system.

Signed-Off-By: Kirill Korotaev <dev@sw.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 27 insertions, 37 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 8e050fa5821..e94ab398b71 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -485,32 +485,6 @@ static void set_sb_syncing(int val)
 	spin_unlock(&sb_lock);
 }
 
-/*
- * Find a superblock with inodes that need to be synced
- */
-static struct super_block *get_super_to_sync(void)
-{
-	struct super_block *sb;
-restart:
-	spin_lock(&sb_lock);
-	sb = sb_entry(super_blocks.prev);
-	for (; sb != sb_entry(&super_blocks); sb = sb_entry(sb->s_list.prev)) {
-		if (sb->s_syncing)
-			continue;
-		sb->s_syncing = 1;
-		sb->s_count++;
-		spin_unlock(&sb_lock);
-		down_read(&sb->s_umount);
-		if (!sb->s_root) {
-			drop_super(sb);
-			goto restart;
-		}
-		return sb;
-	}
-	spin_unlock(&sb_lock);
-	return NULL;
-}
-
 /**
  * sync_inodes - writes all inodes to disk
  * @wait: wait for completion
@@ -530,23 +504,39 @@ restart:
  * outstanding dirty inodes, the writeback goes block-at-a-time within the
  * filesystem's write_inode().  This is extremely slow.
  */
-void sync_inodes(int wait)
+static void __sync_inodes(int wait)
 {
 	struct super_block *sb;
 
-	set_sb_syncing(0);
-	while ((sb = get_super_to_sync()) != NULL) {
-		sync_inodes_sb(sb, 0);
-		sync_blockdev(sb->s_bdev);
-		drop_super(sb);
+	spin_lock(&sb_lock);
+restart:
+	list_for_each_entry(sb, &super_blocks, s_list) {
+		if (sb->s_syncing)
+			continue;
+		sb->s_syncing = 1;
+		sb->s_count++;
+		spin_unlock(&sb_lock);
+		down_read(&sb->s_umount);
+		if (sb->s_root) {
+			sync_inodes_sb(sb, wait);
+			sync_blockdev(sb->s_bdev);
+		}
+		up_read(&sb->s_umount);
+		spin_lock(&sb_lock);
+		if (__put_super_and_need_restart(sb))
+			goto restart;
 	}
+	spin_unlock(&sb_lock);
+}
+
+void sync_inodes(int wait)
+{
+	set_sb_syncing(0);
+	__sync_inodes(0);
+
 	if (wait) {
 		set_sb_syncing(0);
-		while ((sb = get_super_to_sync()) != NULL) {
-			sync_inodes_sb(sb, 1);
-			sync_blockdev(sb->s_bdev);
-			drop_super(sb);
-		}
+		__sync_inodes(1);
 	}
 }