linux-kernel - Re: [PATCH] dma-buf: add attachments empty check for dma_buf

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Date:   Tue, 19 Oct 2021 14:41:14 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     guangming.cao@...iatek.com
Cc:     Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <dri-devel@...ts.freedesktop.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        open list <linux-kernel@...r.kernel.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-arm-kernel@...ts.infradead.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-mediatek@...ts.infradead.org>, wsd_upstream@...iatek.com
Subject: Re: [PATCH] dma-buf: add attachments empty check for dma_buf_release

On Tue, Oct 19, 2021 at 08:23:45PM +0800, guangming.cao@...iatek.com wrote:
> From: Guangming Cao <Guangming.Cao@...iatek.com>
> 
> Since there is no mandatory inspection for attachments in dma_buf_release.
> There will be a case that dma_buf already released but attachment is still
> in use, which can points to the dmabuf, and it maybe cause
> some unexpected issues.
> 
> With IOMMU, when this cases occurs, there will have IOMMU address
> translation fault(s) followed by this warning,
> I think it's useful for dma devices to debug issue.
> 
> Signed-off-by: Guangming Cao <Guangming.Cao@...iatek.com>

This feels a lot like hand-rolling kobject debugging. If you want to do
this then I think adding kobject debug support to
dma_buf/dma_buf_attachment would be better than hand-rolling something
bespoke here.

Also on the patch itself: You don't need the trylock. For correctly
working code non one else can get at the dma-buf, so no locking needed to
iterate through the attachment list. For incorrect code the kernel will be
on fire pretty soon anyway, trying to do locking won't help :-) And
without the trylock we can catch more bugs (e.g. if you also forgot to
unlock and not just forgot to detach).
-Daniel

> ---
>  drivers/dma-buf/dma-buf.c | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 511fe0d217a0..672404857d6a 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -74,6 +74,29 @@ static void dma_buf_release(struct dentry *dentry)
>  	 */
>  	BUG_ON(dmabuf->cb_shared.active || dmabuf->cb_excl.active);
>  
> +	/* attachment check */
> +	if (dma_resv_trylock(dmabuf->resv) && WARN(!list_empty(&dmabuf->attachments),
> +	    "%s err, inode:%08lu size:%08zu name:%s exp_name:%s flags:0x%08x mode:0x%08x, %s\n",
> +	    __func__, file_inode(dmabuf->file)->i_ino, dmabuf->size,
> +	    dmabuf->name, dmabuf->exp_name,
> +	    dmabuf->file->f_flags, dmabuf->file->f_mode,
> +	    "Release dmabuf before detach all attachments, dump attach:\n")) {
> +		int attach_cnt = 0;
> +		dma_addr_t dma_addr;
> +		struct dma_buf_attachment *attach_obj;
> +		/* dump all attachment info */
> +		list_for_each_entry(attach_obj, &dmabuf->attachments, node) {
> +			dma_addr = (dma_addr_t)0;
> +			if (attach_obj->sgt)
> +				dma_addr = sg_dma_address(attach_obj->sgt->sgl);
> +			pr_err("attach[%d]: dev:%s dma_addr:0x%-12lx\n",
> +			       attach_cnt, dev_name(attach_obj->dev), dma_addr);
> +			attach_cnt++;
> +		}
> +		pr_err("Total %d devices attached\n\n", attach_cnt);
> +		dma_resv_unlock(dmabuf->resv);
> +	}
> +
>  	dmabuf->ops->release(dmabuf);
>  
>  	if (dmabuf->resv == (struct dma_resv *)&dmabuf[1])
> -- 
> 2.17.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch