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SubjectRe: [PATCH] dma-buf/poll: Get a file reference for outstanding fence callbacks
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Am 23.07.21 um 09:58 schrieb Michel Dänzer:
> From: Michel Dänzer <mdaenzer@redhat.com>
>
> This makes sure we don't hit the
>
> BUG_ON(dmabuf->cb_in.active || dmabuf->cb_out.active);
>
> in dma_buf_release, which could be triggered by user space closing the
> dma-buf file description while there are outstanding fence callbacks
> from dma_buf_poll.

I was also wondering the same thing while working on this, but then
thought that the poll interface would take care of this.

>
> Cc: stable@vger.kernel.org
> Signed-off-by: Michel Dänzer <mdaenzer@redhat.com>
> ---
> drivers/dma-buf/dma-buf.c | 18 ++++++++++++------
> 1 file changed, 12 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 6c520c9bd93c..ec25498a971f 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -65,12 +65,9 @@ static void dma_buf_release(struct dentry *dentry)
> BUG_ON(dmabuf->vmapping_counter);
>
> /*
> - * Any fences that a dma-buf poll can wait on should be signaled
> - * before releasing dma-buf. This is the responsibility of each
> - * driver that uses the reservation objects.
> - *
> - * If you hit this BUG() it means someone dropped their ref to the
> - * dma-buf while still having pending operation to the buffer.
> + * If you hit this BUG() it could mean:
> + * * There's a file reference imbalance in dma_buf_poll / dma_buf_poll_cb or somewhere else
> + * * dmabuf->cb_in/out.active are non-0 despite no pending fence callback
> */
> BUG_ON(dmabuf->cb_in.active || dmabuf->cb_out.active);
>
> @@ -196,6 +193,7 @@ static loff_t dma_buf_llseek(struct file *file, loff_t offset, int whence)
> static void dma_buf_poll_cb(struct dma_fence *fence, struct dma_fence_cb *cb)
> {
> struct dma_buf_poll_cb_t *dcb = (struct dma_buf_poll_cb_t *)cb;
> + struct dma_buf *dmabuf = container_of(dcb->poll, struct dma_buf, poll);
> unsigned long flags;
>
> spin_lock_irqsave(&dcb->poll->lock, flags);
> @@ -203,6 +201,8 @@ static void dma_buf_poll_cb(struct dma_fence *fence, struct dma_fence_cb *cb)
> dcb->active = 0;
> spin_unlock_irqrestore(&dcb->poll->lock, flags);
> dma_fence_put(fence);
> + /* Paired with get_file in dma_buf_poll */
> + fput(dmabuf->file);

Is calling fput() in interrupt context ok? IIRC that could potentially
sleep.

Regards,
Christian.

> }
>
> static bool dma_buf_poll_shared(struct dma_resv *resv,
> @@ -278,6 +278,9 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll)
> spin_unlock_irq(&dmabuf->poll.lock);
>
> if (events & EPOLLOUT) {
> + /* Paired with fput in dma_buf_poll_cb */
> + get_file(dmabuf->file);
> +
> if (!dma_buf_poll_shared(resv, dcb) &&
> !dma_buf_poll_excl(resv, dcb))
> /* No callback queued, wake up any other waiters */
> @@ -299,6 +302,9 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll)
> spin_unlock_irq(&dmabuf->poll.lock);
>
> if (events & EPOLLIN) {
> + /* Paired with fput in dma_buf_poll_cb */
> + get_file(dmabuf->file);
> +
> if (!dma_buf_poll_excl(resv, dcb))
> /* No callback queued, wake up any other waiters */
> dma_buf_poll_cb(NULL, &dcb->cb);

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