[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>

3 files changed, 96 insertions, 111 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);
         }
 }
 
diff --git a/fs/quota.c b/fs/quota.c
index 3f0333a51a23..f5d1cff55196 100644
--- a/fs/quota.c
+++ b/fs/quota.c
@@ -149,36 +149,6 @@ static int check_quotactl_valid(struct super_block *sb, int type, int cmd, qid_t
         return error;
 }
 
-static struct super_block *get_super_to_sync(int type)
-{
-        struct list_head *head;
-        int cnt, dirty;
-
-restart:
-        spin_lock(&sb_lock);
-        list_for_each(head, &super_blocks) {
-                struct super_block *sb = list_entry(head, struct super_block, s_list);
-
-                /* This test just improves performance so it needn't be reliable... */
-                for (cnt = 0, dirty = 0; cnt < MAXQUOTAS; cnt++)
-                        if ((type == cnt || type == -1) && sb_has_quota_enabled(sb, cnt)
-                            && info_any_dirty(&sb_dqopt(sb)->info[cnt]))
-                                dirty = 1;
-                if (!dirty)
-                        continue;
-                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;
-}
-
 static void quota_sync_sb(struct super_block *sb, int type)
 {
         int cnt;
@@ -219,17 +189,35 @@ static void quota_sync_sb(struct super_block *sb, int type)
 
 void sync_dquots(struct super_block *sb, int type)
 {
+        int cnt, dirty;
+
         if (sb) {
                 if (sb->s_qcop->quota_sync)
                         quota_sync_sb(sb, type);
+                return;
         }
-        else {
-                while ((sb = get_super_to_sync(type)) != NULL) {
-                        if (sb->s_qcop->quota_sync)
-                                quota_sync_sb(sb, type);
-                        drop_super(sb);
-                }
+
+        spin_lock(&sb_lock);
+restart:
+        list_for_each_entry(sb, &super_blocks, s_list) {
+                /* This test just improves performance so it needn't be reliable... */
+                for (cnt = 0, dirty = 0; cnt < MAXQUOTAS; cnt++)
+                        if ((type == cnt || type == -1) && sb_has_quota_enabled(sb, cnt)
+                            && info_any_dirty(&sb_dqopt(sb)->info[cnt]))
+                                dirty = 1;
+                if (!dirty)
+                        continue;
+                sb->s_count++;
+                spin_unlock(&sb_lock);
+                down_read(&sb->s_umount);
+                if (sb->s_root && sb->s_qcop->quota_sync)
+                        quota_sync_sb(sb, type);
+                up_read(&sb->s_umount);
+                spin_lock(&sb_lock);
+                if (__put_super_and_need_restart(sb))
+                        goto restart;
         }
+        spin_unlock(&sb_lock);
 }
 
 /* Copy parameters and call proper function */
diff --git a/fs/super.c b/fs/super.c
index 573bcc81bb82..25bc1ec6bc5d 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -341,20 +341,22 @@ static inline void write_super(struct super_block *sb)
  */
 void sync_supers(void)
 {
-        struct super_block * sb;
-restart:
+        struct super_block *sb;
+
         spin_lock(&sb_lock);
-        sb = sb_entry(super_blocks.next);
-        while (sb != sb_entry(&super_blocks))
+restart:
+        list_for_each_entry(sb, &super_blocks, s_list) {
                 if (sb->s_dirt) {
                         sb->s_count++;
                         spin_unlock(&sb_lock);
                         down_read(&sb->s_umount);
                         write_super(sb);
-                        drop_super(sb);
-                        goto restart;
-                } else
-                        sb = sb_entry(sb->s_list.next);
+                        up_read(&sb->s_umount);
+                        spin_lock(&sb_lock);
+                        if (__put_super_and_need_restart(sb))
+                                goto restart;
+                }
+        }
         spin_unlock(&sb_lock);
 }
 
@@ -381,20 +383,16 @@ void sync_filesystems(int wait)
 
         down(&mutex);           /* Could be down_interruptible */
         spin_lock(&sb_lock);
-        for (sb = sb_entry(super_blocks.next); sb != sb_entry(&super_blocks);
-                        sb = sb_entry(sb->s_list.next)) {
+        list_for_each_entry(sb, &super_blocks, s_list) {
                 if (!sb->s_op->sync_fs)
                         continue;
                 if (sb->s_flags & MS_RDONLY)
                         continue;
                 sb->s_need_sync_fs = 1;
         }
-        spin_unlock(&sb_lock);
 
 restart:
-        spin_lock(&sb_lock);
-        for (sb = sb_entry(super_blocks.next); sb != sb_entry(&super_blocks);
-                        sb = sb_entry(sb->s_list.next)) {
+        list_for_each_entry(sb, &super_blocks, s_list) {
                 if (!sb->s_need_sync_fs)
                         continue;
                 sb->s_need_sync_fs = 0;
@@ -405,8 +403,11 @@ restart:
                 down_read(&sb->s_umount);
                 if (sb->s_root && (wait || sb->s_dirt))
                         sb->s_op->sync_fs(sb, wait);
-                drop_super(sb);
-                goto restart;
+                up_read(&sb->s_umount);
+                /* restart only when sb is no longer on the list */
+                spin_lock(&sb_lock);
+                if (__put_super_and_need_restart(sb))
+                        goto restart;
         }
         spin_unlock(&sb_lock);
         up(&mutex);
@@ -422,21 +423,25 @@ restart:
 
 struct super_block * get_super(struct block_device *bdev)
 {
-        struct list_head *p;
+        struct super_block *sb;
+
         if (!bdev)
                 return NULL;
-rescan:
+
         spin_lock(&sb_lock);
-        list_for_each(p, &super_blocks) {
-                struct super_block *s = sb_entry(p);
-                if (s->s_bdev == bdev) {
-                        s->s_count++;
+rescan:
+        list_for_each_entry(sb, &super_blocks, s_list) {
+                if (sb->s_bdev == bdev) {
+                        sb->s_count++;
                         spin_unlock(&sb_lock);
-                        down_read(&s->s_umount);
-                        if (s->s_root)
-                                return s;
-                        drop_super(s);
-                        goto rescan;
+                        down_read(&sb->s_umount);
+                        if (sb->s_root)
+                                return sb;
+                        up_read(&sb->s_umount);
+                        /* restart only when sb is no longer on the list */
+                        spin_lock(&sb_lock);
+                        if (__put_super_and_need_restart(sb))
+                                goto rescan;
                 }
         }
         spin_unlock(&sb_lock);
@@ -447,20 +452,22 @@ EXPORT_SYMBOL(get_super);
  
 struct super_block * user_get_super(dev_t dev)
 {
-        struct list_head *p;
+        struct super_block *sb;
 
-rescan:
         spin_lock(&sb_lock);
-        list_for_each(p, &super_blocks) {
-                struct super_block *s = sb_entry(p);
-                if (s->s_dev ==  dev) {
-                        s->s_count++;
+rescan:
+        list_for_each_entry(sb, &super_blocks, s_list) {
+                if (sb->s_dev ==  dev) {
+                        sb->s_count++;
                         spin_unlock(&sb_lock);
-                        down_read(&s->s_umount);
-                        if (s->s_root)
-                                return s;
-                        drop_super(s);
-                        goto rescan;
+                        down_read(&sb->s_umount);
+                        if (sb->s_root)
+                                return sb;
+                        up_read(&sb->s_umount);
+                        /* restart only when sb is no longer on the list */
+                        spin_lock(&sb_lock);
+                        if (__put_super_and_need_restart(sb))
+                                goto rescan;
                 }
         }
         spin_unlock(&sb_lock);