Re: [PATCH v2] dma: Add Xilinx AXI Video Direct Memory AccessEngine driver support

[Andy Shevchenko]

On Thu, 2014-01-23 at 14:50 +0100, Lars-Peter Clausen wrote:
> On 01/23/2014 02:38 PM, Shevchenko, Andriy wrote:
> > On Thu, 2014-01-23 at 12:25 +0100, Lars-Peter Clausen wrote:
> >> On 01/22/2014 05:52 PM, Srikanth Thokala wrote:
> > 
> > [...]
> > 
> >>> +	/* Request the interrupt */
> >>> +	chan->irq = irq_of_parse_and_map(node, 0);
> >>> +	err = devm_request_irq(xdev->dev, chan->irq, xilinx_vdma_irq_handler,
> >>> +			       IRQF_SHARED, "xilinx-vdma-controller", chan);
> >>
> >> This is a clasic example of where to not use devm_request_irq. 'chan' is
> >> accessed in the interrupt handler, but if you use devm_request_irq 'chan'
> >> will be freed before the interrupt handler has been released, which means
> >> there is now a race condition where the interrupt handler can access already
> >> freed memory.ta
> > 
> > Could you elaborate this case? As far as I understood managed resources
> > are a kind of stack pile. In this case you have no such condition. Where
> > am I wrong?
> 
> The stacked stuff is only ran after the remove() function. Which means that
> you call dma_async_device_unregister() before the interrupt handler is
> freed. Another issue with the interrupt handler is a bit hidden. The driver
> does not call tasklet_kill in the remove function. Which it should though to
> make sure that the tasklet does not race against the freeing of the memory.
> And in order to make sure that the tasklet is not rescheduled you need to
> free the irq before killing the tasklet, since the interrupt handler
> schedules the tasklet.

So, you mean devm_request_irq() will race in any DMA driver?

I think the proper solution is to disable all device work in
the .remove() and devm will care about resources.
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-- 
Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Intel Finland Oy

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