Re: [Intel-gfx] [PATCH 03/26] dma-buf: use new iterator in dma_resv_copy_fences

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

Am 17.09.21 um 16:35 schrieb Daniel Vetter:
> On Fri, Sep 17, 2021 at 02:34:50PM +0200, Christian König wrote:
>> This makes the function much simpler since the complex
>> retry logic is now handled else where.
>>
>> Signed-off-by: Christian König <christian.koenig@amd.com>
>> ---
>>   drivers/dma-buf/dma-resv.c | 86 ++++++++++++++++----------------------
>>   1 file changed, 35 insertions(+), 51 deletions(-)
>>
>> diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
>> index a3c79a99fb44..406150dea5e4 100644
>> --- a/drivers/dma-buf/dma-resv.c
>> +++ b/drivers/dma-buf/dma-resv.c
>> @@ -426,74 +426,58 @@ EXPORT_SYMBOL_GPL(dma_resv_iter_walk);
>>    */
>>   int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src)
>>   {
>> -	struct dma_resv_list *src_list, *dst_list;
>> -	struct dma_fence *old, *new;
>> -	unsigned int i;
>> +	struct dma_resv_iter cursor;
>> +	struct dma_resv_list *list;
>> +	struct dma_fence *f, *excl;
>>   
>>   	dma_resv_assert_held(dst);
>>   
>> -	rcu_read_lock();
>> -	src_list = dma_resv_shared_list(src);
>> -
>> -retry:
>> -	if (src_list) {
>> -		unsigned int shared_count = src_list->shared_count;
>> -
>> -		rcu_read_unlock();
>> +	list = NULL;
>> +	excl = NULL;
>>   
>> -		dst_list = dma_resv_list_alloc(shared_count);
>> -		if (!dst_list)
>> -			return -ENOMEM;
>> +	rcu_read_lock();
>> +	dma_resv_iter_begin(&cursor, src, true);
>> +	dma_resv_for_each_fence_unlocked(&cursor, f) {
>>   
>> -		rcu_read_lock();
>> -		src_list = dma_resv_shared_list(src);
>> -		if (!src_list || src_list->shared_count > shared_count) {
>> -			kfree(dst_list);
>> -			goto retry;
>> -		}
>> +		if (cursor.is_first) {
> Maybe have a wrapper for this, like dma_resv_iter_is_reset or is_first or
> is_restart (my preference) with some nice docs that this returns true
> everytime we had to restart the sequence?

Exactly that's what I wanted to avoid since the is_first (or whatever) 
function should only be used inside of the dma_resv.c code.

On the other hand I can just make that static here and document that it 
should never be exported.

Christian.

>
> Otherwise I fully agree, this is so much better with all the hairy
> restarting and get_rcu and test_bit shovelled away somewhere.
>
> Either way (but I much prefer a wrapper for is_first):
>
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
>
>> +			dma_resv_list_free(list);
>> +			dma_fence_put(excl);
>>   
>> -		dst_list->shared_count = 0;
>> -		for (i = 0; i < src_list->shared_count; ++i) {
>> -			struct dma_fence __rcu **dst;
>> -			struct dma_fence *fence;
>> +			if (cursor.fences) {
>> +				unsigned int cnt = cursor.fences->shared_count;
>>   
>> -			fence = rcu_dereference(src_list->shared[i]);
>> -			if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
>> -				     &fence->flags))
>> -				continue;
>> +				rcu_read_unlock();
>> +				list = dma_resv_list_alloc(cnt);
>> +				if (!list) {
>> +					dma_resv_iter_end(&cursor);
>> +					return -ENOMEM;
>> +				}
>>   
>> -			if (!dma_fence_get_rcu(fence)) {
>> -				dma_resv_list_free(dst_list);
>> -				src_list = dma_resv_shared_list(src);
>> -				goto retry;
>> -			}
>> +				list->shared_count = 0;
>> +				rcu_read_lock();
>>   
>> -			if (dma_fence_is_signaled(fence)) {
>> -				dma_fence_put(fence);
>> -				continue;
>> +			} else {
>> +				list = NULL;
>>   			}
>> -
>> -			dst = &dst_list->shared[dst_list->shared_count++];
>> -			rcu_assign_pointer(*dst, fence);
>> +			excl = NULL;
>>   		}
>> -	} else {
>> -		dst_list = NULL;
>> -	}
>>   
>> -	new = dma_fence_get_rcu_safe(&src->fence_excl);
>> +		dma_fence_get(f);
>> +		if (dma_resv_iter_is_exclusive(&cursor))
>> +			excl = f;
>> +		else
>> +			RCU_INIT_POINTER(list->shared[list->shared_count++], f);
>> +	}
>> +	dma_resv_iter_end(&cursor);
>>   	rcu_read_unlock();
>>   
>> -	src_list = dma_resv_shared_list(dst);
>> -	old = dma_resv_excl_fence(dst);
>> -
>>   	write_seqcount_begin(&dst->seq);
>> -	/* write_seqcount_begin provides the necessary memory barrier */
>> -	RCU_INIT_POINTER(dst->fence_excl, new);
>> -	RCU_INIT_POINTER(dst->fence, dst_list);
>> +	excl = rcu_replace_pointer(dst->fence_excl, excl, dma_resv_held(dst));
>> +	list = rcu_replace_pointer(dst->fence, list, dma_resv_held(dst));
>>   	write_seqcount_end(&dst->seq);
>>   
>> -	dma_resv_list_free(src_list);
>> -	dma_fence_put(old);
>> +	dma_resv_list_free(list);
>> +	dma_fence_put(excl);
>>   
>>   	return 0;
>>   }
>> -- 
>> 2.25.1
>>