Re: [RFC][PATCH 1/2] dma-buf: add importer private data to attachment

[Maarten Lankhorst <maarten.lankhorst@canonical.com>]

Op 07-06-13 04:32, 김승우 schreef:
> Hello Maarten,
>
> On 2013년 06월 05일 22:23, Maarten Lankhorst wrote:
>> Op 31-05-13 10:54, Seung-Woo Kim schreef:
>>> dma-buf attachment has only exporter private data, but importer private data
>>> can be useful for importer especially to re-import the same dma-buf.
>>> To use importer private data in attachment of the device, the function to
>>> search attachment in the attachment list of dma-buf is also added.
>>>
>>> Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
>>> ---
>>>  drivers/base/dma-buf.c  |   31 +++++++++++++++++++++++++++++++
>>>  include/linux/dma-buf.h |    4 ++++
>>>  2 files changed, 35 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/drivers/base/dma-buf.c b/drivers/base/dma-buf.c
>>> index 08fe897..a1eaaf2 100644
>>> --- a/drivers/base/dma-buf.c
>>> +++ b/drivers/base/dma-buf.c
>>> @@ -259,6 +259,37 @@ err_attach:
>>>  EXPORT_SYMBOL_GPL(dma_buf_attach);
>>>  
>>>  /**
>>> + * dma_buf_get_attachment - Get attachment with the device from dma_buf's
>>> + * attachments list
>>> + * @dmabuf:	[in]	buffer to find device from.
>>> + * @dev:	[in]	device to be found.
>>> + *
>>> + * Returns struct dma_buf_attachment * attaching the device; may return
>>> + * negative error codes.
>>> + *
>>> + */
>>> +struct dma_buf_attachment *dma_buf_get_attachment(struct dma_buf *dmabuf,
>>> +						  struct device *dev)
>>> +{
>>> +	struct dma_buf_attachment *attach;
>>> +
>>> +	if (WARN_ON(!dmabuf || !dev))
>>> +		return ERR_PTR(-EINVAL);
>>> +
>>> +	mutex_lock(&dmabuf->lock);
>>> +	list_for_each_entry(attach, &dmabuf->attachments, node) {
>>> +		if (attach->dev == dev) {
>>> +			mutex_unlock(&dmabuf->lock);
>>> +			return attach;
>>> +		}
>>> +	}
>>> +	mutex_unlock(&dmabuf->lock);
>>> +
>>> +	return ERR_PTR(-ENODEV);
>>> +}
>>> +EXPORT_SYMBOL_GPL(dma_buf_get_attachment);
>> NAK in any form..
>>
>> Spot the race condition between dma_buf_get_attachment and dma_buf_attach....
> Both dma_buf_get_attachment and dma_buf_attach has mutet with
> dmabuf->lock, and dma_buf_get_attachment is used for get attachment from
> same device before calling dma_buf_attach.

hint: what happens if 2 threads do this:

if (IS_ERR(attach = dma_buf_get_attachment(buf, dev)))
attach = dma_buf_attach(buf, dev);

There really is no correct usecase for this kind of thing, so please don't do it.

> While, dma_buf_detach can removes attachment because it does not have
> ref count. So importer should check ref count in its importer private
> data before calling dma_buf_detach if the importer want to use
> dma_buf_get_attachment.
>
> And dma_buf_get_attachment is for the importer, so exporter attach and
> detach callback operation should not call it as like exporter detach
> callback operation should not call dma_buf_attach if you mean this kind
> of race.
>
> If you have other considerations here, please describe more specifically.
>
> Thanks and Best Regards,
> - Seung-Woo Kim
>
>> ~Maarten
>>
>>