[PATCH] md: Small fixes for timely writing of md superblocks.

From: NeilBrown <neilb@cse.unsw.edu.au>

Currently a raid0 superblock is only written when the array is stopped, so a
crash between creation and stop can lose your data.

This patch marks a superblock 'dirty' at creation and forces a dirty
superblock to be written when the array is started.

Previously we would prod the per-array thread at this point, but as it avoids
certain chores when the array is locked, and the array is locked at this
point, that isn't guaranteed to do the right thing.  Instead we prod the
thread whenever the array is unlocked.

Finally, only write the superblock at array stop if it is needed to mark the
array as 'clean'.  raid0 which is never dirty, doesn't need this.

drivers/md/md.c

@@ -246,6 +246,9 @@ static inline int mddev_trylock(mddev_t * mddev)
 static inline void mddev_unlock(mddev_t * mddev)
 {
 	up(&mddev->reconfig_sem);
+
+	if (mddev->thread)
+		md_wakeup_thread(mddev->thread);
 }
 
 mdk_rdev_t * find_rdev_nr(mddev_t *mddev, int nr)
@@ -1626,7 +1629,10 @@ static int do_md_run(mddev_t * mddev)
 	mddev->in_sync = 1;
 	
 	set_bit(MD_RECOVERY_NEEDED, &mddev->recovery);
-	md_wakeup_thread(mddev->thread);
+	
+	if (mddev->sb_dirty)
+		md_update_sb(mddev);
+
 	set_capacity(disk, mddev->array_size<<1);
 
 	/* If we call blk_queue_make_request here, it will
@@ -1717,7 +1723,7 @@ static int do_md_stop(mddev_t * mddev, int ro)
 			if (mddev->ro)
 				mddev->ro = 0;
 		}
-		if (mddev->raid_disks) {
+		if (!mddev->in_sync) {
 			/* mark array as shutdown cleanly */
 			mddev->in_sync = 1;
 			md_update_sb(mddev);
@@ -2290,7 +2296,7 @@ static int hot_add_disk(mddev_t * mddev, dev_t dev)
 /*
  * set_array_info is used two different ways
  * The original usage is when creating a new array.
- * In this usage, raid_disks is > = and it together with
+ * In this usage, raid_disks is > 0 and it together with
  *  level, size, not_persistent,layout,chunksize determine the
  *  shape of the array.
  *  This will always create an array with a type-0.90.0 superblock.
@@ -2341,6 +2347,7 @@ static int set_array_info(mddev_t * mddev, mdu_array_info_t *info)
 
 	mddev->max_disks     = MD_SB_DISKS;
 
+	mddev->sb_dirty      = 1;
 
 	/*
 	 * Generate a 128 bit UUID
@@ -3104,6 +3111,10 @@ void md_write_end(mddev_t *mddev)
 
 static inline void md_enter_safemode(mddev_t *mddev)
 {
+	if (!mddev->safemode) return;
+	if (mddev->safemode == 2 &&
+	    (atomic_read(&mddev->writes_pending) || mddev->in_sync))
+		return; /* avoid the lock */
 	mddev_lock_uninterruptible(mddev);
 	if (mddev->safemode && !atomic_read(&mddev->writes_pending) &&
 	    !mddev->in_sync && mddev->recovery_cp == MaxSector) {
@@ -3124,8 +3135,7 @@ void md_handle_safemode(mddev_t *mddev)
 		mddev->safemode = 2;
 		flush_signals(current);
 	}
-	if (mddev->safemode)
-		md_enter_safemode(mddev);
+	md_enter_safemode(mddev);
 }
 
 
@@ -3313,8 +3323,7 @@ static void md_do_sync(mddev_t *mddev)
 			mddev->recovery_cp = MaxSector;
 	}
 
-	if (mddev->safemode)
-		md_enter_safemode(mddev);
+	md_enter_safemode(mddev);
  skip:
 	mddev->curr_resync = 0;
 	set_bit(MD_RECOVERY_DONE, &mddev->recovery);