Re: [PATCH 01/12] dma-buf: Add reference counting to dma_resv

[Danilo Krummrich]

On Sat Jul 11, 2026 at 3:27 AM CEST, Matthew Brost wrote:
> On Fri, Jul 10, 2026 at 08:52:41PM +0200, Christian König wrote:
>> This provides clearer ownership semantics and makes the code more
>> maintainable by removing the embedded allocation hack.
>> 
>
> This looks a lot better to me. In particular, I agree with the last
> sentence in the commit message.

I have to disagree with this, it is the opposite. As long as the struct
dma_resv::allocated fields and the corresponding semantics exists, this does
result into less clear ownership semantics.

When the dma_resv is embedded in another object the reference count becomes
meaningless. If the object embedding the dma_resv is freed it doesn't matter
whether I have a reference count, it would a UAF regardless.

It is misleading (and hence error prone) to have an API where one can obtain a
reference count of an object where the underlying memory can be freed regardless
of the obtained reference count.

A refernece count represents a shared ownership model, which is undermined if
the underlying memory is not owned by the reference count.

That said, I don't mind the reference count, but we can't mix up exclusive
ownership (embedding a structure) and shared ownership (reference count).

>> +static void dma_resv_release(struct kref *kref)
>>  {
>> -	/*
>> -	 * This object should be dead and all references must have
>> -	 * been released to it, so no need to be protected with rcu.
>> -	 */
>> +	struct dma_resv *obj = container_of(kref, struct dma_resv, refcount);
>> +
>>  	dma_resv_list_free(rcu_dereference_protected(obj->fences, true));
>>  	ww_mutex_destroy(&obj->lock);
>> +	if (obj->allocated)
>> +		kfree(obj);
>> +}