From dc750422170a563c7a81f6e49d36bb02c62ae37f Mon Sep 17 00:00:00 2001 From: Shivank Garg Date: Sat, 22 Aug 2026 19:22:06 +0000 Subject: [PATCH] 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. This call will delay for a grace period with dma_list_mutex held, which is safe and only teardown path is delayed. Fixes: 2ba05622b8b1 ("dmaengine: provide a common 'issue_pending_all' implementation") Suggested-by: Sashiko Link: https://sashiko.dev/#/patchset/20260526-dmaengine-kref-fix-v2-0-3df60afac01d@amd.com Reviewed-by: Frank Li Reviewed-by: Logan Gunthorpe Signed-off-by: Shivank Garg Link: https://patch.msgid.link/20260822-dmaengine-kref-fix-v5-4-d4a4ee47d927@amd.com Signed-off-by: Vinod Koul --- drivers/dma/dmaengine.c | 1 + 1 file changed, 1 insertion(+) diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c index 872ae0d2ed95..c71763047126 100644 --- a/drivers/dma/dmaengine.c +++ b/drivers/dma/dmaengine.c @@ -428,6 +428,7 @@ static void dma_device_release(struct kref *ref) list_del_rcu(&device->global_node); dma_channel_rebalance(); + synchronize_rcu(); if (device->device_release) device->device_release(device);