[PATCH] Fix race in do_get_write_access()

  attached patch should fix the following race:
     Proc 1                               Proc 2

     __flush_batch()
       ll_rw_block()
                                        do_get_write_access()
					   lock_buffer
                                             jh is only waiting for checkpoint
					     -> b_transaction == NULL ->
					     do nothing
                                           unlock_buffer
    test_set_buffer_locked()
    test_clear_buffer_dirty()
                                           __journal_file_buffer()
                                        change the data
    submit_bh()

and we have sent wrong data to disk...  We now clean the dirty buffer flag
under buffer lock in all cases and hence we know that whenever a buffer is
starting to be journaled we either finish the pending write-out before
attaching a buffer to a transaction or we won't write the buffer until the
transaction is going to be committed.

The test in jbd_unexpected_dirty_buffer() is redundant - remove it.
Furthermore we have to clear the buffer dirty bit under the buffer lock to
prevent races with buffer write-out (and hence prevent returning a buffer with
IO happening).
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/jbd/transaction.c

@@ -490,10 +490,8 @@ void journal_unlock_updates (journal_t *journal)
  */
 static void jbd_unexpected_dirty_buffer(struct journal_head *jh)
 {
-	struct buffer_head *bh = jh2bh(jh);
 	int jlist;
 
-	if (buffer_dirty(bh)) {
 	/* If this buffer is one which might reasonably be dirty
 	 * --- ie. data, or not part of this journal --- then
 	 * we're OK to leave it alone, but otherwise we need to
@@ -503,10 +501,10 @@ static void jbd_unexpected_dirty_buffer(struct journal_head *jh)
 
 	if (jlist == BJ_Metadata || jlist == BJ_Reserved ||
 	    jlist == BJ_Shadow || jlist == BJ_Forget) {
-			if (test_clear_buffer_dirty(jh2bh(jh))) {
-				set_bit(BH_JBDDirty, &jh2bh(jh)->b_state);
-			}
-		}
+		struct buffer_head *bh = jh2bh(jh);
+
+		if (test_clear_buffer_dirty(bh))
+			set_buffer_jbddirty(bh);
 	}
 }
 
@@ -574,10 +572,15 @@ repeat:
 			if (jh->b_next_transaction)
 				J_ASSERT_JH(jh, jh->b_next_transaction ==
 							transaction);
+		}
+		/*
+		 * In any case we need to clean the dirty flag and we must
+		 * do it under the buffer lock to be sure we don't race
+		 * with running write-out.
+		 */
 		JBUFFER_TRACE(jh, "Unexpected dirty buffer");
 		jbd_unexpected_dirty_buffer(jh);
  	}
- 	}
 
 	unlock_buffer(bh);