Re: [PATCH] dma-buf: refcount the attachment for cache_sgt_mapping

[Koenig, Christian]

Am 12.06.19 um 03:22 schrieb Nicolin Chen:
> Commit f13e143e7444 ("dma-buf: start caching of sg_table objects v2")
> added a support of caching the sgt pointer into an attach pointer to
> let users reuse the sgt pointer without another mapping. However, it
> might not totally work as most of dma-buf callers are doing attach()
> and map_attachment() back-to-back, using drm_prime.c for example:
>      drm_gem_prime_import_dev() {
>          attach = dma_buf_attach() {
>              /* Allocating a new attach */
>              attach = kzalloc();
>              /* .... */
>              return attach;
>          }
>          dma_buf_map_attachment(attach, direction) {
>              /* attach->sgt would be always empty as attach is new */
>              if (attach->sgt) {
>                  /* Reuse attach->sgt */
>              }
>              /* Otherwise, map it */
>              attach->sgt = map();
>          }
>      }
>
> So, for a cache_sgt_mapping use case, it would need to get the same
> attachment pointer in order to reuse its sgt pointer. So this patch
> adds a refcount to the attach() function and lets it search for the
> existing attach pointer by matching the dev pointer.

I don't think that this is a good idea.

We use sgt caching as workaround for locking order problems and want to 
remove it again in the long term.

So what is the actual use case of this?

Regards,
Christian.

>
> Signed-off-by: Nicolin Chen <nicoleotsuka@xxxxxxxxx>
> ---
>   drivers/dma-buf/dma-buf.c | 23 +++++++++++++++++++++++
>   include/linux/dma-buf.h   |  2 ++
>   2 files changed, 25 insertions(+)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index f4104a21b069..d0260553a31c 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -559,6 +559,21 @@ struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
>   	if (WARN_ON(!dmabuf || !dev))
>   		return ERR_PTR(-EINVAL);
>   
> +	/* cache_sgt_mapping requires to reuse the same attachment pointer */
> +	if (dmabuf->ops->cache_sgt_mapping) {
> +		mutex_lock(&dmabuf->lock);
> +
> +		/* Search for existing attachment and increase its refcount */
> +		list_for_each_entry(attach, &dmabuf->attachments, node) {
> +			if (dev != attach->dev)
> +				continue;
> +			atomic_inc_not_zero(&attach->refcount);
> +			goto unlock_attach;
> +		}
> +
> +		mutex_unlock(&dmabuf->lock);
> +	}
> +
>   	attach = kzalloc(sizeof(*attach), GFP_KERNEL);
>   	if (!attach)
>   		return ERR_PTR(-ENOMEM);
> @@ -575,6 +590,9 @@ struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
>   	}
>   	list_add(&attach->node, &dmabuf->attachments);
>   
> +	atomic_set(&attach->refcount, 1);
> +
> +unlock_attach:
>   	mutex_unlock(&dmabuf->lock);
>   
>   	return attach;
> @@ -599,6 +617,11 @@ void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *attach)
>   	if (WARN_ON(!dmabuf || !attach))
>   		return;
>   
> +	/* Decrease the refcount for cache_sgt_mapping use cases */
> +	if (dmabuf->ops->cache_sgt_mapping &&
> +	    atomic_dec_return(&attach->refcount))
> +		return;
> +
>   	if (attach->sgt)
>   		dmabuf->ops->unmap_dma_buf(attach, attach->sgt, attach->dir);
>   
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> index 8a327566d7f4..65f12212ca2e 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -333,6 +333,7 @@ struct dma_buf {
>    * @dev: device attached to the buffer.
>    * @node: list of dma_buf_attachment.
>    * @sgt: cached mapping.
> + * @refcount: refcount of the attachment for the same device.
>    * @dir: direction of cached mapping.
>    * @priv: exporter specific attachment data.
>    *
> @@ -350,6 +351,7 @@ struct dma_buf_attachment {
>   	struct device *dev;
>   	struct list_head node;
>   	struct sg_table *sgt;
> +	atomic_t refcount;
>   	enum dma_data_direction dir;
>   	void *priv;
>   };