Commit 5826cade authored Oct 13, 2006 by David Gibson Committed by Jeff Garzik Oct 21, 2006

[PATCH] ibmveth: Fix index increment calculation

On Thu, Oct 12, 2006 at 06:22:14PM +1000, David Gibson wrote:
> Your recent ibmveth commit, 751ae21c
> ("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 751ae21c 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>

parent cde49b05

drivers/net/ibmveth.c

+6 −4

Original line number	Diff line number	Diff line
		@@ -212,8 +212,8 @@ static void ibmveth_replenish_buffer_pool(struct ibmveth_adapter *adapter, struc
		break;
		}

		free_index = pool->consumer_index++ % pool->size;
		pool->consumer_index = free_index;
		free_index = pool->consumer_index;
		pool->consumer_index = (pool->consumer_index + 1) % pool->size;
		index = pool->free_map[free_index];

		ibmveth_assert(index != IBM_VETH_INVALID_MAP);
		@@ -329,8 +329,10 @@ static void ibmveth_remove_buffer_from_pool(struct ibmveth_adapter *adapter, u64
		adapter->rx_buff_pool[pool].buff_size,
		DMA_FROM_DEVICE);

		free_index = adapter->rx_buff_pool[pool].producer_index++ % adapter->rx_buff_pool[pool].size;
		adapter->rx_buff_pool[pool].producer_index = free_index;
		free_index = adapter->rx_buff_pool[pool].producer_index;
		adapter->rx_buff_pool[pool].producer_index
		= (adapter->rx_buff_pool[pool].producer_index + 1)
		% adapter->rx_buff_pool[pool].size;
		adapter->rx_buff_pool[pool].free_map[free_index] = index;

		mb();