Fix race in create_empty_buffers() vs __set_page_dirty_buffers()

Nick Piggin noticed this (very unlikely) race between setting a page
dirty and creating the buffers for it - we need to hold the mapping
private_lock until we've set the page dirty bit in order to make sure
that create_empty_buffers() might not build up a set of buffers without
the dirty bits set when the page is dirty.

I doubt anybody has ever hit this race (and it didn't solve the issue
Nick was looking at), but as Nick says: "Still, it does appear to solve
a real race, which we should close."

Acked-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/buffer.c

@@ -760,15 +760,9 @@ EXPORT_SYMBOL(mark_buffer_dirty_inode);
  * If warn is true, then emit a warning if the page is not uptodate and has
  * not been truncated.
  */
-static int __set_page_dirty(struct page *page,
+static void __set_page_dirty(struct page *page,
 		struct address_space *mapping, int warn)
 {
-	if (unlikely(!mapping))
-		return !TestSetPageDirty(page);
-
-	if (TestSetPageDirty(page))
-		return 0;
-
 	spin_lock_irq(&mapping->tree_lock);
 	if (page->mapping) {	/* Race with truncate? */
 		WARN_ON_ONCE(warn && !PageUptodate(page));
@@ -785,8 +779,6 @@ static int __set_page_dirty(struct page *page,
 	}
 	spin_unlock_irq(&mapping->tree_lock);
 	__mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
-
-	return 1;
 }
 
 /*
@@ -816,6 +808,7 @@ static int __set_page_dirty(struct page *page,
  */
 int __set_page_dirty_buffers(struct page *page)
 {
+	int newly_dirty;
 	struct address_space *mapping = page_mapping(page);
 
 	if (unlikely(!mapping))
@@ -831,9 +824,12 @@ int __set_page_dirty_buffers(struct page *page)
 			bh = bh->b_this_page;
 		} while (bh != head);
 	}
+	newly_dirty = !TestSetPageDirty(page);
 	spin_unlock(&mapping->private_lock);
 
-	return __set_page_dirty(page, mapping, 1);
+	if (newly_dirty)
+		__set_page_dirty(page, mapping, 1);
+	return newly_dirty;
 }
 EXPORT_SYMBOL(__set_page_dirty_buffers);
 
@@ -1262,8 +1258,11 @@ void mark_buffer_dirty(struct buffer_head *bh)
 			return;
 	}
 
-	if (!test_set_buffer_dirty(bh))
-		__set_page_dirty(bh->b_page, page_mapping(bh->b_page), 0);
+	if (!test_set_buffer_dirty(bh)) {
+		struct page *page = bh->b_page;
+		if (!TestSetPageDirty(page))
+			__set_page_dirty(page, page_mapping(page), 0);
+	}
 }
 
 /*