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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;
    > -
    > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
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    > More majordomo info at http://vger.kernel.org/majordomo-info.html
    > 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
    --
    ==============================================
    Mr. Shannon Nelson Parents can't afford to be squeamish.
    -
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