linux-kernel - Re: [PATCH] dma-buf: Fix missing excl fence waiting

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Date:   Sun, 23 Feb 2020 13:04:15 +0100
From:   Christian König <christian.koenig@....com>
To:     "Pan, Xinhui" <Xinhui.Pan@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>
Cc:     "sumit.semwal@...aro.org" <sumit.semwal@...aro.org>,
        "daniel.vetter@...ll.ch" <daniel.vetter@...ll.ch>
Subject: Re: [PATCH] dma-buf: Fix missing excl fence waiting

Am 23.02.20 um 12:56 schrieb Pan, Xinhui:
> If shared fence list is not empty, even we want to test all fences, excl fence is ignored.
> That is abviously wrong, so fix it.

Yeah that is a known issue and I completely agree with you, but other 
disagree.

See the shared fences are meant to depend on the exclusive fence. So all 
shared fences must finish only after the exclusive one has finished as well.

The problem now is that for error handling this isn't necessary true. In 
other words when a shared fence completes with an error it is perfectly 
possible that he does this before the exclusive fence is finished.

I'm trying to convince Daniel that this is a problem for years :)

Regards,
Christian.

>
> Signed-off-by: xinhui pan <xinhui.pan@....com>
> ---
>   drivers/dma-buf/dma-resv.c | 9 +++++----
>   1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
> index 4264e64788c4..44dc64c547c6 100644
> --- a/drivers/dma-buf/dma-resv.c
> +++ b/drivers/dma-buf/dma-resv.c
> @@ -632,14 +632,14 @@ static inline int dma_resv_test_signaled_single(struct dma_fence *passed_fence)
>    */
>   bool dma_resv_test_signaled_rcu(struct dma_resv *obj, bool test_all)
>   {
> -	unsigned seq, shared_count;
> +	unsigned int seq, shared_count, left;
>   	int ret;
>   
>   	rcu_read_lock();
>   retry:
>   	ret = true;
>   	shared_count = 0;
> -	seq = read_seqcount_begin(&obj->seq);
> +	left = seq = read_seqcount_begin(&obj->seq);
>   
>   	if (test_all) {
>   		unsigned i;
> @@ -647,7 +647,7 @@ bool dma_resv_test_signaled_rcu(struct dma_resv *obj, bool test_all)
>   		struct dma_resv_list *fobj = rcu_dereference(obj->fence);
>   
>   		if (fobj)
> -			shared_count = fobj->shared_count;
> +			left = shared_count = fobj->shared_count;
>   
>   		for (i = 0; i < shared_count; ++i) {
>   			struct dma_fence *fence = rcu_dereference(fobj->shared[i]);
> @@ -657,13 +657,14 @@ bool dma_resv_test_signaled_rcu(struct dma_resv *obj, bool test_all)
>   				goto retry;
>   			else if (!ret)
>   				break;
> +			left--;
>   		}
>   
>   		if (read_seqcount_retry(&obj->seq, seq))
>   			goto retry;
>   	}
>   
> -	if (!shared_count) {
> +	if (!left) {
>   		struct dma_fence *fence_excl = rcu_dereference(obj->fence_excl);
>   
>   		if (fence_excl) {