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

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Date:   Fri,  2 Dec 2022 10:25:06 -0800
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     fenghua.yu@...el.com, dave.jiang@...el.com, vkoul@...nel.org,
        dmaengine@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH 3/3] dmaengine: idxd: Do not call DMX TX callbacks during workqueue disable

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;
 	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) {
 		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