linux-kernel - Re: [PATCH] dma-buf/poll: Get a file reference for outstanding fence callbacks

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Date:   Fri, 23 Jul 2021 10:04:58 +0200
From:   Christian König <christian.koenig@....com>
To:     Michel Dänzer <michel@...nzer.net>
Cc:     Sumit Semwal <sumit.semwal@...aro.org>,
        dri-devel@...ts.freedesktop.org, linux-media@...r.kernel.org,
        linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma-buf/poll: Get a file reference for outstanding fence
 callbacks

Am 23.07.21 um 09:58 schrieb Michel Dänzer:
> From: Michel Dänzer <mdaenzer@...hat.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@...r.kernel.org
> Signed-off-by: Michel Dänzer <mdaenzer@...hat.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);