Re: [Linaro-mm-sig] [PATCH] dma-buf: add attachments empty check for dma_buf_release

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

Am 19.10.21 um 14:41 schrieb Daniel Vetter:
> On Tue, Oct 19, 2021 at 08:23:45PM +0800, guangming.cao@mediatek.com wrote:
>> From: Guangming Cao <Guangming.Cao@mediatek.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@mediatek.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.

Well I would call that overkill.

>
> 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).

You also don't need the WARN(!list_empty...) because a few line below we 
already have a "WARN_ON(!list_empty(&dmabuf->attachments));".

Christian.

> -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
>>