[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 8e050fa58218..e94ab398b717 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);
         }
 }