[PATCH] dma-debug: Do not add notifier when dma debugging is disabled.

[Shaun Ruffell <sruffell@digium.com>]

If CONFIG_HAVE_DMA_API_DEBUG is defined and "dma_debug=off" is
specified on the kernel command line, when you detach a driver from a
device you can cause the following NULL pointer dereference:

BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<c0580d35>] dma_debug_device_change+0x5d/0x117

The problem is that the dma_debug_device_change notifier function is
added to the bus notifier chain even though the dma_entry_hash array
was never initialized.  If dma debugging is disabled, this patch both
prevents dma_debug_device_change notifiers from being added to the
chain, and additionally ensures that the dma_entry_hash array is always
initialized to a valid state.

Signed-off-by: Shaun Ruffell <sruffell@digium.com>
---
 lib/dma-debug.c |    9 ++++++---
 1 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/lib/dma-debug.c b/lib/dma-debug.c
index d9b08e0..a504ead 100644
--- a/lib/dma-debug.c
+++ b/lib/dma-debug.c
@@ -697,6 +697,9 @@ void dma_debug_add_bus(struct bus_type *bus)
 {
 	struct notifier_block *nb;
 
+	if (global_disable)
+		return;
+
 	nb = kzalloc(sizeof(struct notifier_block), GFP_KERNEL);
 	if (nb == NULL) {
 		pr_err("dma_debug_add_bus: out of memory\n");
@@ -715,14 +718,14 @@ void dma_debug_init(u32 num_entries)
 {
 	int i;
 
-	if (global_disable)
-		return;
-
 	for (i = 0; i < HASH_SIZE; ++i) {
 		INIT_LIST_HEAD(&dma_entry_hash[i].list);
 		spin_lock_init(&dma_entry_hash[i].lock);
 	}
 
+	if (global_disable)
+		return;
+
 	if (dma_debug_fs_init() != 0) {
 		pr_err("DMA-API: error creating debugfs entries - disabling\n");
 		global_disable = true;
-- 
1.6.5.2