[rfc][patch 3/3] fs: sys_sync fix

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s_syncing livelock avoidance was breaking data integrity guarantee of sys_sync,
by allowing sys_sync to skip writing or waiting for superblocks if there is a
concurrent sys_sync happening.

This livelock avoidance is much less important now that we don't have the
get_super_to_sync() call after every sb that we sync. This was replaced by
__put_super_and_need_restart, which only triggers on unmount.

---
Index: linux-2.6/fs/fs-writeback.c
===================================================================
--- linux-2.6.orig/fs/fs-writeback.c
+++ linux-2.6/fs/fs-writeback.c
@@ -645,18 +645,6 @@ void sync_inodes_sb(struct super_block *
 	sync_sb_inodes(sb, &wbc);
 }
 
-/*
- * Rather lame livelock avoidance.
- */
-static void set_sb_syncing(int val)
-{
-	struct super_block *sb;
-	spin_lock(&sb_lock);
-	list_for_each_entry_reverse(sb, &super_blocks, s_list)
-		sb->s_syncing = val;
-	spin_unlock(&sb_lock);
-}
-
 /**
  * sync_inodes - writes all inodes to disk
  * @wait: wait for completion
@@ -683,9 +671,6 @@ static void __sync_inodes(int wait)
 	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);
@@ -703,13 +688,10 @@ restart:
 
 void sync_inodes(int wait)
 {
-	set_sb_syncing(0);
 	__sync_inodes(0);
 
-	if (wait) {
-		set_sb_syncing(0);
+	if (wait)
 		__sync_inodes(1);
-	}
 }
 
 /**
Index: linux-2.6/include/linux/fs.h
===================================================================
--- linux-2.6.orig/include/linux/fs.h
+++ linux-2.6/include/linux/fs.h
@@ -1122,7 +1122,6 @@ struct super_block {
 	struct rw_semaphore	s_umount;
 	struct mutex		s_lock;
 	int			s_count;
-	int			s_syncing;
 	int			s_need_sync_fs;
 	atomic_t		s_active;
 #ifdef CONFIG_SECURITY
--
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