Filesystems appear to be using ->dirty_inode, expecting that the dirtying
operating is done and visible to all CPUs (eg. setting private inode dirty
bits, without any barriers themselves). So release the dirty "critical
section" with a barrier before calling ->dirty_inode.

Cost is not significantly changed, because we're just moving the barrier.
Those filesystems that do use ->dirty_inode should have to care slightly
less about barriers, which is a good thing.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>

Index: linux-2.6/fs/fs-writeback.c
===================================================================
--- linux-2.6.orig/fs/fs-writeback.c	2010-11-19 16:47:00.000000000 +1100
+++ linux-2.6/fs/fs-writeback.c	2010-11-19 16:49:39.000000000 +1100
@@ -934,6 +934,15 @@ void __mark_inode_dirty(struct inode *in
 	bool wakeup_bdi = false;
 
 	/*
+	 * Make sure that changes are seen by all cpus before we test i_state
+	 * or mark anything as being dirty. Ie. all dirty state should be
+	 * written to the inode and visible. Like an "unlock" operation, the
+	 * mark_inode_dirty call must "release" our ordering window that is
+	 * opened when we started modifying the inode.
+	 */
+	smp_mb();
+
+	/*
 	 * Don't do this for I_DIRTY_PAGES - that doesn't actually
 	 * dirty the inode itself
 	 */
@@ -942,12 +951,6 @@ void __mark_inode_dirty(struct inode *in
 			sb->s_op->dirty_inode(inode);
 	}
 
-	/*
-	 * make sure that changes are seen by all cpus before we test i_state
-	 * -- mikulas
-	 */
-	smp_mb();
-
 	/* avoid the locking if we can */
 	if ((inode->i_state & flags) == flags)
 		return;


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