From 5826cade4341a6298eb10d476dccc5f403ca7ad8 Mon Sep 17 00:00:00 2001 From: David Gibson Date: Fri, 13 Oct 2006 14:20:59 +1000 Subject: [PATCH] [PATCH] ibmveth: Fix index increment calculation On Thu, Oct 12, 2006 at 06:22:14PM +1000, David Gibson wrote: > Your recent ibmveth commit, 751ae21c6cd1493e3d0a4935b08fb298b9d89773 > ("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 751ae21c6cd1493e3d0a4935b08fb298b9d89773 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 Signed-off-by: Jeff Garzik --- drivers/net/ibmveth.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) diff --git a/drivers/net/ibmveth.c b/drivers/net/ibmveth.c index 2802db23d3..44c9f993dc 100644 --- a/drivers/net/ibmveth.c +++ b/drivers/net/ibmveth.c @@ -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(); -- 2.39.5