Re: [PATCH] dmaengine: Free channel ID after device_unregister()

[Yao Yuan]

On Tue, Nov 10, 2026 at 06:16:07PM +0800, zhejun_huang@xxxxxxxxxxxxxxxxx wrote:
> From: Zhejun Huang <zhejun_huang@xxxxxxxxxxxxxxxxx>
>
> __dma_async_device_channel_unregister() frees the channel ID before calling
> device_unregister(). Between ida_free() and device_del() a concurrent
> __dma_async_device_channel_register() on the same device can reallocate the
> ID and try to register a device with the same name while the old one is
> still present, failing with -EEXIST.
>
> This is reachable in practice: idxd calls these functions from its
> per-workqueue probe and remove callbacks, which are only serialized by a
> per-wq mutex. Enabling one wq while another is being removed therefore
> runs them concurrently on the same dma_device.
>
> Move ida_free() after device_unregister() so that device_del() removes the
> name before the ID becomes reusable.
>
> Fixes: 0821009445a8 ("dmaengine: fix channel index enumeration")
> Cc: <stable@xxxxxxxxxxxxxxx> # 5.7+
> Signed-off-by: Zhejun Huang <zhejun_huang@xxxxxxxxxxxxxxxxx>
> ---
>  drivers/dma/dmaengine.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
> index 34be53487a21..2642abf105d7 100644
> --- a/drivers/dma/dmaengine.c
> +++ b/drivers/dma/dmaengine.c
> @@ -1171,8 +1171,8 @@ static void __dma_async_device_channel_unregister(struct dma_device *device,
>  	device->chancnt--;
>  	chan->chan_dev->chan = NULL;
>  	mutex_unlock(&dma_list_mutex);
> -	ida_free(&device->chan_ida, chan->chan_id);
>  	device_unregister(dmaengine_chan_dev(chan));
> +	ida_free(&device->chan_ida, chan->chan_id);

Reviewed-by: Yuan Yao <yaoyuan@xxxxxxxxxxxxxxxxx>

>  	free_percpu(chan->local);
>  }
>
>
> base-commit: d48723711bae9b8cdc56719c448ba42fef28cbee
> --
> 2.50.1 (Apple Git-155)
>