[PATCH] __block_write_full_page race fix

When running
	fsstress -v -d $DIR/tmp -n 1000 -p 1000 -l 2
on an ext2 filesystem with 1024 byte block size, on SMP i386 with 4096 byte
page size over loopback to an image file on a tmpfs filesystem, I would
very quickly hit
	BUG_ON(!buffer_async_write(bh));
in fs/buffer.c:end_buffer_async_write

It seems that more than one request would be submitted for a given bh
at a time.

What would happen is the following:
2 threads doing __mpage_writepages on the same page.
Thread 1 - lock the page first, and enter __block_write_full_page.
Thread 1 - (eg.) mark_buffer_async_write on the first 2 buffers.
Thread 1 - set page writeback, unlock page.
Thread 2 - lock page, wait on page writeback
Thread 1 - submit_bh on the first 2 buffers.
=> both requests complete, none of the page buffers are async_write,
   end_page_writeback is called.
Thread 2 - wakes up. enters __block_write_full_page.
Thread 2 - mark_buffer_async_write on (eg.) the last buffer
Thread 1 - finds the last buffer has async_write set, submit_bh on that.
Thread 2 - submit_bh on the last buffer.
=> oops.

So change __block_write_full_page to explicitly keep track of the last bh
we need to issue, so we don't touch anything after issuing the last
request.
Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/buffer.c

@@ -1751,7 +1751,7 @@ static int __block_write_full_page(struct inode *inode, struct page *page,
 	int err;
 	sector_t block;
 	sector_t last_block;
-	struct buffer_head *bh, *head;
+	struct buffer_head *bh, *head, *last_bh = NULL;
 	int nr_underway = 0;
 
 	BUG_ON(!PageLocked(page));
@@ -1809,7 +1809,6 @@ static int __block_write_full_page(struct inode *inode, struct page *page,
 	} while (bh != head);
 
 	do {
-		get_bh(bh);
 		if (!buffer_mapped(bh))
 			continue;
 		/*
@@ -1827,6 +1826,8 @@ static int __block_write_full_page(struct inode *inode, struct page *page,
 		}
 		if (test_clear_buffer_dirty(bh)) {
 			mark_buffer_async_write(bh);
+			get_bh(bh);
+			last_bh = bh;
 		} else {
 			unlock_buffer(bh);
 		}
@@ -1845,10 +1846,13 @@ static int __block_write_full_page(struct inode *inode, struct page *page,
 		if (buffer_async_write(bh)) {
 			submit_bh(WRITE, bh);
 			nr_underway++;
-		}
 			put_bh(bh);
+			if (bh == last_bh)
+				break;
+		}
 		bh = next;
 	} while (bh != head);
+	bh = head;
 
 	err = 0;
 done:
@@ -1887,10 +1891,11 @@ recover:
 	bh = head;
 	/* Recovery: lock and submit the mapped buffers */
 	do {
-		get_bh(bh);
 		if (buffer_mapped(bh) && buffer_dirty(bh)) {
 			lock_buffer(bh);
 			mark_buffer_async_write(bh);
+			get_bh(bh);
+			last_bh = bh;
 		} else {
 			/*
 			 * The buffer may have been set dirty during
@@ -1909,10 +1914,13 @@ recover:
 			clear_buffer_dirty(bh);
 			submit_bh(WRITE, bh);
 			nr_underway++;
-		}
 			put_bh(bh);
+			if (bh == last_bh)
+				break;
+		}
 		bh = next;
 	} while (bh != head);
+	bh = head;
 	goto done;
 }