Re: [BUG] media: xilinx: dma - stop_streaming does not wait for a running DMA completion callback

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

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On Sat, 31 Oct 2026 20:26:53 +0000, Samarth Narang <samarth.colleges@gmail.com> wrote:
> Hi,
> 
> I think xvip_dma_stop_streaming() can race with an in-progress DMA
> completion callback, potentially causing a buffer to be returned twice
> or accessed after it has been released. The line numbers below refer to
> mainline at 36aad21d50d1.
> 
> In drivers/media/platform/xilinx/xilinx-dma.c, xvip_dma_stop_streaming()
> calls dmaengine_terminate_all() (line 442) before taking queued_lock and
> returning the buffers on queued_bufs with VB2_BUF_STATE_ERROR (lines
> 449-454). Meanwhile, xvip_dma_complete() takes the same lock, removes
> the completed buffer from the list, and then calls vb2_buffer_done()
> with VB2_BUF_STATE_DONE after releasing the lock (lines 283-296).
> 
> The Xilinx DMA provider invokes completion callbacks from a tasklet
> after releasing the channel lock. Although xilinx_dma_terminate_all()
> halts the channel and frees its descriptor lists, it does not wait for
> a callback that is already running. That synchronization is handled by
> xilinx_dma_synchronize(), which calls tasklet_kill()
> (drivers/dma/xilinx/xilinx_dma.c, lines 2739-2743), but is not invoked
> by dmaengine_terminate_all().
> 
> If a completion callback is already running when streaming is stopped,
> the two paths can race over the same buffer. If stop_streaming()
> acquires queued_lock first, it may remove the buffer and return it with
> VB2_BUF_STATE_ERROR before the callback attempts to remove the same list
> entry and return it again with VB2_BUF_STATE_DONE. Even when the
> callback removes the entry first, it may still be using the buffer after
> stop_streaming() returns, at which point videobuf2 may release it. This
> is a callback synchronization issue rather than an outstanding hardware
> DMA transfer, since the engine itself has already been halted.
> 
> Since stop_streaming() can sleep, I believe dmaengine_terminate_sync()
> would address the race, following the approach used for stm32-dcmi in
> ab32b2c3636b ("media: stm32-dcmi: remove deprecated
> dmaengine_terminate_all()"):
> 
> --- a/drivers/media/platform/xilinx/xilinx-dma.c
> +++ b/drivers/media/platform/xilinx/xilinx-dma.c
> @@ -439,7 +439,7 @@ static void xvip_dma_stop_streaming(struct vb2_queue *vq)
>  	xvip_pipeline_set_stream(pipe, false);
>  
>  	/* Stop and reset the DMA engine. */
> -	dmaengine_terminate_all(dma->dma);
> +	dmaengine_terminate_sync(dma->dma);
>  
>  	/* Cleanup the pipeline and mark it as being stopped. */
>  	xvip_pipeline_cleanup(pipe);
> 
> There is also a related question about the error path in
> xvip_dma_start_streaming() (lines 422-427), which returns previously
> queued buffers with VB2_BUF_STATE_QUEUED without terminating the DMA
> channel. As far as I can tell, their descriptors remain pending and
> could be issued by a subsequent dma_async_issue_pending(). Is that the
> intended behavior?
> 
> This issue was identified by a research static analyzer for DMA
> lifecycle ordering and subsequently checked against the client and
> provider implementations. I have not reproduced it on hardware, as I do
> not have access to a Xilinx video pipeline and QEMU does not emulate the
> relevant VDMA or video IP. The proposed change has been compile-tested
> on arm64 with W=1.
> 
> If someone with suitable hardware can confirm the race or test the
> change, I would be happy to submit it as a proper patch.
> 
> Thanks,
> Samarth
> 
>