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

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Date:   Tue, 25 Feb 2020 18:23:55 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Christian König <christian.koenig@....com>
Cc:     "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>,
        "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

On Sun, Feb 23, 2020 at 01:04:15PM +0100, Christian König wrote:
> 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 :)

I thought the consensus is that reasonable gpu schedulers and gpu reset
code should try to make really, really sure it only completes stuff in
sequence? That's at least my take away from the syncobj timeline
discussion, where you convinced me we shouldn't just crash&burn.

I think as long as your scheduler is competent and your gpu reset tries to
limit damage (i.e. kill offending ctx terminally, mark everything else
that didn't complete for re-running) we should end up with everything
completing in sequence. I guess if you do kill a lot more stuff, then
you'd have to push these through your scheduler as dummy jobs, i.e. they
still wait for their dependencies, but then all they do is set the
dma_fence error and complete it. Maybe something the common scheduler
could do.
-Daniel

> 
> 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) {
> 

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