[PATCH] DMAENGINE: too many kref_put calls

[PATCH] DMAENGINE: too many kref_put calls

[Shannon Nelson]

When a channel is removed from dmaengine, too many kref_put() calls
are made and the device removal happens too soon, usually causing
a panic.

Cc: Haavard Skinnemoen <hskinnemoen@atmel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
---

 drivers/dma/dmaengine.c |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)

diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
index 84257f7..245da53 100644
--- a/drivers/dma/dmaengine.c
+++ b/drivers/dma/dmaengine.c
@@ -134,8 +134,7 @@ static void dma_async_device_cleanup(struct kref *kref);
 
 static void dma_dev_release(struct device *dev)
 {
-	struct dma_chan *chan = to_dma_chan(dev);
-	kref_put(&chan->device->refcount, dma_async_device_cleanup);
+	return;
 }
 
 static struct class dma_devclass = {

Re: [PATCH] DMAENGINE: too many kref_put calls

[Al Viro]

On Fri, Oct 26, 2007 at 04:56:48PM -0700, Shannon Nelson wrote:

>  static void dma_dev_release(struct device *dev)
>  {
> -	struct dma_chan *chan = to_dma_chan(dev);
> -	kref_put(&chan->device->refcount, dma_async_device_cleanup);
> +	return;
>  }

Practically guaranteed to be broken.  Empty ->release() is almost certain
to mean that you can get operations on already freed object.

Re: [PATCH] DMAENGINE: too many kref_put calls

[Haavard Skinnemoen]

On Fri, 26 Oct 2007 16:56:48 -0700
Shannon Nelson <shannon.nelson@intel.com> wrote:

> @@ -134,8 +134,7 @@ static void dma_async_device_cleanup(struct kref
> *kref); 
>  static void dma_dev_release(struct device *dev)
>  {
> -	struct dma_chan *chan = to_dma_chan(dev);
> -	kref_put(&chan->device->refcount, dma_async_device_cleanup);
> +	return;
>  }

Hmm...what prevents dma_chan_cleanup() from being called while someone
still holds a reference to the "class" device? This will allow the
module removal to proceed, the device to be freed, and things will blow
up...

I think my patch is better. By taking two references, the order in
which the cleanup functions are called doesn't matter -- both have to
complete before the module removal is allowed to proceed.

Håvard