[PATCH] sync_sb_inodes sync hang

[Hugh Dickins]

2.6.3-mm UP without PREEMPT easily hangs looping around pdflush's
sync_sb_inodes, failing the down_read_trylock in do_writepages,
but giving the concurrent sync no chance to complete: just try
while : ; do echo $SECONDS; sync; cp /etc/termcap .; done

I think sync_sb_inodes is the only loop vulnerable to that
change in do_writepages, so site the cond_resched() here.

Hugh

--- 2.6.3-mm3/fs/fs-writeback.c	2004-02-23 12:51:49.000000000 +0000
+++ linux/fs/fs-writeback.c	2004-02-24 16:14:49.000000000 +0000
@@ -326,6 +326,7 @@ sync_sb_inodes(struct super_block *sb, s
 			writeback_release(bdi);
 		spin_unlock(&inode_lock);
 		iput(inode);
+		cond_resched();
 		spin_lock(&inode_lock);
 		if (wbc->nr_to_write <= 0)
 			break;

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