On Thu, Oct 12, 2006 at 06:22:14PM +1000, David Gibson wrote:
> Your recent ibmveth commit, 751ae21c6c
> ("fix int rollover panic"), causes a rapid oops on my test machine
> (POWER5 LPAR).
>
> I've bisected it down to that commit, but am still investigating the
> cause of the crash itself.
Found the problem, I believe: an object lesson in the need for great
caution using ++.
[...]
@@ -213,6 +213,7 @@ static void ibmveth_replenish_buffer_poo
}
free_index = pool->consumer_index++ % pool->size;
+ pool->consumer_index = free_index;
index = pool->free_map[free_index];
ibmveth_assert(index != IBM_VETH_INVALID_MAP);
Since the ++ is used as post-increment, the increment is not included
in free_index, and so the added line effectively reverts the
increment. The produced_index side has an analagous bug.
The following change corrects this:
The recent commit 751ae21c6c introduced
a bug in the producer/consumer index calculation in the ibmveth driver
- incautious use of the post-increment ++ operator resulted in an
increment being immediately reverted. This patch corrects the logic.
Without this patch, the driver oopses almost immediately after
activation on at least some machines.
Signed-off-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>
5826cade43