[PATCH] mmotm: fix hang at startup

[Hugh Dickins <hughd@google.com>]

Yesterday's mmotm hangs at startup, and with lockdep it reports:
BUG: spinlock recursion on CPU#1, blkid/284 - with bdi_lock_two()
called from bdev_inode_switch_bdi() in the backtrace.  It appears
that this function is sometimes called with new the same as old.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
Fix to
writeback-split-inode_wb_list_lock-into-bdi_writebacklist_lock.patch

 fs/block_dev.c |    2 ++
 1 file changed, 2 insertions(+)

--- 2.6.39-rc5-mm1/fs/block_dev.c	2011-04-29 18:20:09.183314733 -0700
+++ linux/fs/block_dev.c	2011-04-30 17:55:45.718785263 -0700
@@ -57,6 +57,8 @@ static void bdev_inode_switch_bdi(struct
 {
 	struct backing_dev_info *old = inode->i_data.backing_dev_info;
 
+	if (dst == old)
+		return;
 	bdi_lock_two(&old->wb, &dst->wb);
 	spin_lock(&inode->i_lock);
 	inode->i_data.backing_dev_info = dst;

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