[PATCH 01/15] iio: buffer: let the block keep its DMA fence

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Josef Bacik <josef@toxicpanda.com>

iio_buffer_enqueue_dmabuf() hands a plain pointer to the fence to the
buffer implementation, which keeps it in block->fence, and since the fix
for the fence leak it drops the initial reference right after
dma_resv_add_fence(). The only reference left is the one of the
reservation object.

That one does not last until the fence is signalled. dma_resv_add_fence()
replaces and puts a fence as soon as a later fence of the same context and
the same usage is added. All the fences of an attachment share a context,
and an enqueue on an output buffer only waits for the writers, not for the
read fence of the enqueue before it. A second IIO_BUFFER_DMABUF_ENQUEUE of
a buffer which is still queued is refused by iio_dma_can_enqueue_block(),
but by then its fence has taken the place of the first one. When the DMA
is done, iio_buffer_signal_dmabuf_done() signals a fence which has been
freed and schedules the work item in it.

The leak came from the reference which iio_buffer_signal_dmabuf_done()
takes and iio_buffer_cleanup() drops: with it nobody dropped the initial
one. Do not take it, and leave the initial reference to the block until
the cleanup.

Fixes: a09399935508 ("iio: buffer: Fix DMA fence leak in iio_buffer_enqueue_dmabuf()")
Cc: stable <stable@kernel.org>
Cc: David Lechner <dlechner@baylibre.com>
Cc: "Nuno Sá" <nuno.sa@analog.com>
Cc: Andy Shevchenko <andy@kernel.org>
Cc: "Benoît Monin" <benoit.monin@bootlin.com>
Cc: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/iio/industrialio-buffer.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c
index 4294101810fd..a2386dba497c 100644
--- a/drivers/iio/industrialio-buffer.c
+++ b/drivers/iio/industrialio-buffer.c
@@ -1931,9 +1931,14 @@ static int iio_buffer_enqueue_dmabuf(struct iio_dev_buffer_pair *ib,
 	if (ret)
 		goto err_queue_unlock;
 
+	/*
+	 * The reservation object takes a reference of its own, and drops it as
+	 * soon as a later fence of this attachment takes the place of this
+	 * one. The initial reference stays with the block, which points to the
+	 * fence until iio_buffer_cleanup() has run.
+	 */
 	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();
@@ -1989,18 +1994,12 @@ void iio_buffer_signal_dmabuf_done(struct dma_fence *fence, int ret)
 		container_of(fence, struct iio_dma_fence, base);
 	bool cookie = dma_fence_begin_signalling();
 
-	/*
-	 * Get a reference to the fence, so that it's not freed as soon as
-	 * it's signaled.
-	 */
-	dma_fence_get(fence);
-
 	fence->error = ret;
 	dma_fence_signal(fence);
 	dma_fence_end_signalling(cookie);
 
 	/*
-	 * The fence will be unref'd in iio_buffer_cleanup.
+	 * The reference of the block will be dropped in iio_buffer_cleanup.
 	 * It can't be done here, as the unref functions might try to lock the
 	 * resv object, which can deadlock.
 	 */

-- 
2.56.0