[PATCH v3 3/3] dmaengine: wait for RCU readers before releasing dma_device

dma_issue_pending_all() walks the dma_device_list with
list_for_each_entry_rcu() under rcu_read_lock(). dma_device_release()
unlinks the device with list_del_rcu() and then calls
device->device_release() (which in many drivers, such as plx_dma.c,
directly calls kfree()).

Because there is no grace period between unlinking the device and
freeing it, concurrent RCU readers in dma_issue_pending_all() can
access the device after it has been freed.

The lockless walk originally relied on clients holding a dmaengine
reference to pin the provider module, and therefore the device, for as
long as they might traverse the list. Commit 8ad342a86359 ("dmaengine:
Add reference counting to dma_device struct") decoupled the dma_device
lifetime from the module reference, so the device can now be released
while a reader is still walking the list.

Add synchronize_rcu() before the device is freed, so RCU readers are
guaranteed to have finished. Keep it unconditional: providers that do
not implement device_release() free the device themselves once
dma_async_device_unregister() returns, so they need the same grace
period.

Fixes: 2ba05622b8b1 ("dmaengine: provide a common 'issue_pending_all' implementation")
Suggested-by: Sashiko <[email protected]>
Link: https://sashiko.dev/#/patchset/[email protected]
Signed-off-by: Shivank Garg <[email protected]>
---
 drivers/dma/dmaengine.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
index bf491eb10596..29b128aa0bea 100644
--- a/drivers/dma/dmaengine.c
+++ b/drivers/dma/dmaengine.c
@@ -429,6 +429,12 @@ static void dma_device_release(struct kref *ref)
 	list_del_rcu(&device->global_node);
 	dma_channel_rebalance();
 
+	/*
+	 * Wait for RCU readers (e.g. dma_issue_pending_all()) that may still
+	 * be traversing dma_device_list before the device is freed.
+	 */
+	synchronize_rcu();
+
 	if (device->device_release)
 		device->device_release(device);
 }

-- 
2.43.0