[PATCH] iio: buffer: Fix DMA fence leak in iio_buffer_enqueue_dmabuf()

[Benoît Monin]

iio_buffer_enqueue_dmabuf() allocates a struct iio_dma_fence (104 bytes,
kmalloc-128) via kmalloc_obj()+dma_fence_init(), which sets the initial
kref to 1.  It then calls dma_resv_add_fence() which takes a second
reference (kref=2), and stores a raw pointer in block->fence.

On the success path the function returns without calling dma_fence_put()
to release the initial reference, so every buffer enqueue permanently
leaks one kmalloc-128 allocation.

The iio_buffer_cleanup() work item only releases the temporary reference
taken during completion signalling by iio_buffer_signal_dmabuf_done();
the initial reference from dma_fence_init() is never released.

With four iio_rwdev instances at 240kHz and 512 samples per buffer,
this produces ~1875 kmalloc-128 allocations per second matching the
observed slab growth exactly. A test with ftrace confirmed that the
dma_fence_destroy event was never triggered.

Fix by calling dma_fence_put() after dma_resv_add_fence(), transferring
ownership of the fence to the DMA reservation object. The DMA fence then
gets properly discarded after being signalled.

Fixes: 3e26d9f08fbe0 ("iio: core: Add new DMABUF interface infrastructure")
Originally-by: James Nuss <jamesnuss@xxxxxxxxxxxxxx>
Signed-off-by: Benoît Monin <benoit.monin@xxxxxxxxxxx>
---
 drivers/iio/industrialio-buffer.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c
index 46f36a6ed271..5c3df993bea2 100644
--- a/drivers/iio/industrialio-buffer.c
+++ b/drivers/iio/industrialio-buffer.c
@@ -1909,6 +1909,7 @@ static int iio_buffer_enqueue_dmabuf(struct iio_dev_buffer_pair *ib,
 
 	dma_resv_add_fence(dmabuf->resv, &fence->base,
 			   dma_to_ram ? DMA_RESV_USAGE_WRITE : DMA_RESV_USAGE_READ);
+	dma_fence_put(&fence->base);
 	dma_resv_unlock(dmabuf->resv);
 
 	cookie = dma_fence_begin_signalling();

---
base-commit: 7aaa8047eafd0bd628065b15757d9b48c5f9c07d
change-id: 20260401-iio-dma-fence-5f1ceba11ef5

Best regards,
--  
Benoît Monin, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com