Re: [PATCH 2/4] dma-buf: stop using the dmabuf->lock so much

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Oct 21, 2019 at 01:15:22PM +0200, Christian König wrote:
> The attachment list is now protected by the dma_resv object.
> So we can drop holding this lock to allow concurrent attach
> and detach operations.
> 
> Signed-off-by: Christian König <christian.koenig@amd.com>
> ---
>  drivers/dma-buf/dma-buf.c | 16 ----------------
>  1 file changed, 16 deletions(-)
> 
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 753be84b5fd6..c736e67ae1a1 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -685,8 +685,6 @@ dma_buf_dynamic_attach(struct dma_buf *dmabuf, struct device *dev,
>  	attach->dmabuf = dmabuf;
>  	attach->dynamic_mapping = dynamic_mapping;
>  
> -	mutex_lock(&dmabuf->lock);
> -
>  	if (dmabuf->ops->attach) {
>  		ret = dmabuf->ops->attach(dmabuf, attach);
>  		if (ret)
> @@ -696,8 +694,6 @@ dma_buf_dynamic_attach(struct dma_buf *dmabuf, struct device *dev,
>  	list_add(&attach->node, &dmabuf->attachments);
>  	dma_resv_unlock(dmabuf->resv);
>  
> -	mutex_unlock(&dmabuf->lock);

This changes the rules, now ->attach/->detach and the list manipulation
aren't done under the same lock anymore. I don't think this matters, but
imo good to mention in the commit message.

> -
>  	/* When either the importer or the exporter can't handle dynamic
>  	 * mappings we cache the mapping here to avoid issues with the
>  	 * reservation object lock.
> @@ -726,7 +722,6 @@ dma_buf_dynamic_attach(struct dma_buf *dmabuf, struct device *dev,
>  
>  err_attach:
>  	kfree(attach);
> -	mutex_unlock(&dmabuf->lock);
>  	return ERR_PTR(ret);
>  
>  err_unlock:
> @@ -776,14 +771,12 @@ void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *attach)
>  			dma_resv_unlock(attach->dmabuf->resv);
>  	}
>  
> -	mutex_lock(&dmabuf->lock);
>  	dma_resv_lock(dmabuf->resv, NULL);
>  	list_del(&attach->node);
>  	dma_resv_unlock(dmabuf->resv);
>  	if (dmabuf->ops->detach)
>  		dmabuf->ops->detach(dmabuf, attach);
>  
> -	mutex_unlock(&dmabuf->lock);
>  	kfree(attach);
>  }
>  EXPORT_SYMBOL_GPL(dma_buf_detach);
> @@ -1247,14 +1240,6 @@ static int dma_buf_debug_show(struct seq_file *s, void *unused)
>  		   "size", "flags", "mode", "count", "ino");
>  
>  	list_for_each_entry(buf_obj, &db_list.head, list_node) {
> -		ret = mutex_lock_interruptible(&buf_obj->lock);
> -
> -		if (ret) {
> -			seq_puts(s,
> -				 "\tERROR locking buffer object: skipping\n");
> -			continue;
> -		}
> -

This will mildly conflict with the revised version of patch 1 (since the
dma_resv_lock needs to be here).

With both nits addressed:

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

>  		seq_printf(s, "%08zu\t%08x\t%08x\t%08ld\t%s\t%08lu\t%s\n",
>  				buf_obj->size,
>  				buf_obj->file->f_flags, buf_obj->file->f_mode,
> @@ -1307,7 +1292,6 @@ static int dma_buf_debug_show(struct seq_file *s, void *unused)
>  
>  		count++;
>  		size += buf_obj->size;
> -		mutex_unlock(&buf_obj->lock);
>  	}
>  
>  	seq_printf(s, "\nTotal %d objects, %zu bytes\n", count, size);
> -- 
> 2.17.1
> 

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