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SubjectRe: [PATCH] DMA: Fix broken device refcounting
On 10/27/07, Dan Williams <dan.j.williams@intel.com> wrote:
> On Sat, 2007-10-27 at 06:49 -0700, Haavard Skinnemoen wrote:
> > On Fri, 26 Oct 2007 09:36:17 -0700
> > Dan Williams <dan.j.williams@intel.com> wrote:
> >
> > > @@ -221,7 +220,6 @@ void dma_chan_cleanup(struct kref *kref)
> > > {
> > > struct dma_chan *chan = container_of(kref, struct dma_chan, refcount);
> > > chan->device->device_free_chan_resources(chan);
> > > - kref_put(&chan->device->refcount, dma_async_device_cleanup);
> > > }
> > > EXPORT_SYMBOL(dma_chan_cleanup);
> >
> > While I can't see any problems with the rest of the patch, I think this
> > part is wrong for the same reasons removing the kref_put() from the
> > class device cleanup function is. I don't see any constraint that
> > guarantees that dma_chan_cleanup() will always be called before
> > dma_dev_release(), which means that "chan" may have been freed before
> > this function gets a chance to run. Please correct me if I'm wrong.
>
> Absolutely right, the driver, not dmaengine, frees the memory so there
> must be a per channel reference on the device to hold off the driver's
> remove routine.
> >
> > Håvard
>
> So how about this...
>
> ---snip---
> dmaengine: Fix broken device refcounting
>
> From: Haavard Skinnemoen <hskinnemoen@atmel.com>
>
> When a DMA device is unregistered, its reference count is decremented
> twice for each channel: Once dma_class_dev_release() and once in
> dma_chan_cleanup(). This may result in the DMA device driver's
> remove() function completing before all channels have been cleaned
> up, causing lots of use-after-free fun.
>
> Fix it by incrementing the device's reference count twice for each
> channel during registration.
>
> Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
> [dan.j.williams@intel.com: kill unnecessary client refcounting]
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
>
> drivers/dma/dmaengine.c | 17 ++++++-----------
> 1 files changed, 6 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
> index 84257f7..ec7e871 100644
> --- a/drivers/dma/dmaengine.c
> +++ b/drivers/dma/dmaengine.c
> @@ -186,10 +186,9 @@ static void dma_client_chan_alloc(struct dma_client *client)
> /* we are done once this client rejects
> * an available resource
> */
> - if (ack == DMA_ACK) {
> + if (ack == DMA_ACK)
> dma_chan_get(chan);
> - kref_get(&device->refcount);
> - } else if (ack == DMA_NAK)
> + else if (ack == DMA_NAK)
> return;
> }
> }
> @@ -276,11 +275,8 @@ static void dma_clients_notify_removed(struct dma_chan *chan)
> /* client was holding resources for this channel so
> * free it
> */
> - if (ack == DMA_ACK) {
> + if (ack == DMA_ACK)
> dma_chan_put(chan);
> - kref_put(&chan->device->refcount,
> - dma_async_device_cleanup);
> - }
> }
>
> mutex_unlock(&dma_list_mutex);
> @@ -320,11 +316,8 @@ void dma_async_client_unregister(struct dma_client *client)
> ack = client->event_callback(client, chan,
> DMA_RESOURCE_REMOVED);
>
> - if (ack == DMA_ACK) {
> + if (ack == DMA_ACK)
> dma_chan_put(chan);
> - kref_put(&chan->device->refcount,
> - dma_async_device_cleanup);
> - }
> }
>
> list_del(&client->global_node);
> @@ -401,6 +394,8 @@ int dma_async_device_register(struct dma_device *device)
> goto err_out;
> }
>
> + /* One for the channel, one of the class device */
> + kref_get(&device->refcount);
> kref_get(&device->refcount);
> kref_init(&chan->refcount);
> chan->slow_ref = 0;
> -
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> Please read the FAQ at http://www.tux.org/lkml/
>

Thanks - when I get back in tomorrow morning I'll test this to see
that it gets rid of the panic that I've been getting.

sln
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