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dmaengine: Fix device kref underflow in dma_chan_put()

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dma_chan_get() takes chan->device->ref only on the slow path:

	/* no kref on fast path */
	if (chan->client_count) {
		__module_get(owner);
		chan->client_count++;
		return 0;
	}
	if (!try_module_get(owner))
		return -ENODEV;
	if (!dma_device_get(chan->device)) { // calls kref_get_unless_zero()

dma_chan_put() drops the ref unconditionally, so every fast-path
get/put pair drops one extra device reference.

The bug fires when two conditions hold together: a non-private
provider has a persistent client holding chan->client_count > 0
and another client cycles dmaengine_get()/dmaengine_put().
When the kref hits zero, the subsequent dma_find_channel() returns
NULL even though the provider module is still loaded.

Fix this by dropping device->ref only on the last put, matching the
single slow-path get.

Fixes: 8ad342a863 ("dmaengine: Add reference counting to dma_device struct")
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Shivank Garg <shivankg@amd.com>
Link: https://patch.msgid.link/20260822-dmaengine-kref-fix-v5-2-d4a4ee47d927@amd.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>
This commit is contained in:
Shivank Garg 2026-08-22 19:22:04 +00:00 committed by Vinod Koul
parent 424103642d
commit 44dab65906
1 changed files with 3 additions and 1 deletions
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4
drivers/dma/dmaengine.c
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@ -520,7 +520,9 @@ static void dma_chan_put(struct dma_chan *chan)
chan->route_data = NULL;
}
dma_device_put(chan->device);
/* This channel is not in use anymore, drop the device ref */
if (!chan->client_count)
dma_device_put(chan->device);
module_put(dma_chan_to_owner(chan));
}