linux-kernel - RE: [PATCH 3/3] dmaengine: idxd: Do not call DMX TX callbacks during workqueue disable

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Date:   Fri, 2 Dec 2022 21:12:51 +0000
From:   "Yu, Fenghua" <fenghua.yu@...el.com>
To:     "Chatre, Reinette" <reinette.chatre@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "vkoul@...nel.org" <vkoul@...nel.org>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 3/3] dmaengine: idxd: Do not call DMX TX callbacks during
 workqueue disable

Hi, Reinette,

> On driver unload any pending descriptors are flushed and pending DMA
> descriptors are explicitly completed:
> idxd_dmaengine_drv_remove() ->
> 	drv_disable_wq() ->
> 		idxd_wq_free_irq() ->
> 			idxd_flush_pending_descs() ->
> 				idxd_dma_complete_txd()
> 
> With this done during driver unload any remaining descriptor is likely stuck and
> can be dropped. Even so, the descriptor may still have a callback set that could
> no longer be accessible. An example of such a problem is when the dmatest fails
> and the dmatest module is unloaded. The failure of dmatest leaves descriptors
> with dma_async_tx_descriptor::callback pointing to code that no longer exist.
> This causes a page fault as below at the time the IDXD driver is unloaded when it
> attempts to run the callback:
>  BUG: unable to handle page fault for address: ffffffffc0665190
>  #PF: supervisor instruction fetch in kernel mode
>  #PF: error_code(0x0010) - not-present page
> 
> Fix this by clearing the callback pointers on the transmit descriptors only when
> workqueue is disabled.
> 
> Signed-off-by: Reinette Chatre <reinette.chatre@...el.com>
> ---
> 
> History of refactoring made the Fixes: hard to identify by me.
> 
>  drivers/dma/idxd/device.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/dma/idxd/device.c b/drivers/dma/idxd/device.c index
> b4d7bb923a40..2ac71a34fa34 100644
> --- a/drivers/dma/idxd/device.c
> +++ b/drivers/dma/idxd/device.c
> @@ -1156,6 +1156,7 @@ int idxd_device_load_config(struct idxd_device *idxd)
> 
>  static void idxd_flush_pending_descs(struct idxd_irq_entry *ie)  {
> +	struct dma_async_tx_descriptor *tx;

Nitpicking. It's better to move this line to below:

>  	struct idxd_desc *desc, *itr;
>  	struct llist_node *head;
>  	LIST_HEAD(flist);
> @@ -1175,6 +1176,15 @@ static void idxd_flush_pending_descs(struct
> idxd_irq_entry *ie)
>  	list_for_each_entry_safe(desc, itr, &flist, list) {

here?
+	struct dma_async_tx_descriptor *tx;

>  		list_del(&desc->list);
>  		ctype = desc->completion->status ?
> IDXD_COMPLETE_NORMAL : IDXD_COMPLETE_ABORT;
> +		/*
> +		 * wq is being disabled. Any remaining descriptors are
> +		 * likely to be stuck and can be dropped. callback could
> +		 * point to code that is no longer accessible, for example
> +		 * if dmatest module has been unloaded.
> +		 */
> +		tx = &desc->txd;
> +		tx->callback = NULL;
> +		tx->callback_result = NULL;
>  		idxd_dma_complete_txd(desc, ctype, true);
>  	}
>  }
> --
> 2.34.1

Reviewed-by: Fenghua Yu <fenghua.yu@...el.com>

Thanks.

-Fenghua