On Mon, Jun 14, 2021 at 07:15:44PM +0200, Christian König wrote:
> Am 11.06.21 um 16:55 schrieb Daniel Vetter:
> > On Fri, Jun 11, 2021 at 04:53:11PM +0200, Christian König wrote:
> > >
> > > Am 11.06.21 um 16:47 schrieb Daniel Vetter:
> > > > On Fri, Jun 11, 2021 at 02:02:57PM +0200, Christian König wrote:
> > > > > As the name implies if testing all fences is requested we
> > > > > should indeed test all fences and not skip the exclusive
> > > > > one because we see shared ones.
> > > > >
> > > > > Signed-off-by: Christian König <christian.koe...@amd.com>
> > > > Hm I thought we've had the rule that when both fences exist, then
> > > > collectively the shared ones must signale no earlier than the exclusive
> > > > one.
> > > >
> > > > That's at least the contract we've implemented in dma_resv.h. But I've
> > > > also found a bunch of drivers who are a lot more yolo on this.
> > > >
> > > > I think there's a solid case here to just always take all the fences if
> > > > we
> > > > ask for all the shared ones, but if we go that way then I'd say
> > > > - clear kerneldoc patch to really hammer this in (currently we're not
> > > > good
> > > > at all in this regard)
> > > > - going through drivers a bit to check for this (I have some of that
> > > > done
> > > > already in my earlier series, need to respin it and send it out)
> > > >
> > > > But I'm kinda not seeing why this needs to be in this patch series here.
> > > You mentioned that this is a problem in the last patch and if you ask me
> > > that's just a bug or at least very inconsistent.
> > >
> > > See dma_resv_wait_timeout() always waits for all fences, including the
> > > exclusive one even if shared ones are present. But
> > > dma_resv_test_signaled()
> > > ignores the exclusive one if shared ones are present.
> > Hm the only one I thought I've mentioned is that dma_buf_poll doesn't use
> > dma_fence_get_rcu_safe where I think it should. Different problem. I think
> > this is one you spotted.
> >
> > > The only other driver I could find trying to make use of this is nouveau
> > > and
> > > I already provided a fix for this as well.
> > i915 also does this, and I think I've found a few more.
> >
> > > I just think that this is the more defensive approach to fix this and have
> > > at least the core functions consistent on the handling.
> > Oh fully agree, it's just current dma_resv docs aren't the greatest, and
> > hacking on semantics without updating the docs isn't great. Especially
> > when it's ad-hoc.
>
> Well when the requirement that shared fences should always signal after the
> exclusive fence is not documented anywhere then I would say that it is
> naturally allowed to just add any fence to the list of shared fence and any
> code assuming something else is just broken and need fixing.
That's not what I meant. I thought the rule is that the shared fences
_together_ need to signal after the exclusive ones. Not each individual
one.
This means that if you have both exclusive fences and shared fences, and
you want to wait for just the shared fences, then you can ignore the
exclusive ones.
You have a patch series floating around which "fixes" this, but I think
it's imcomplete. And I'm pretty sure it's a change of defacto rules, since
not obeying this breaks a bunch of existing code (as you've noticed).
-Daniel
>
> Christian.
>
> > -Daniel
> >
> > > Christian.
> > >
> > > > -Daniel
> > > >
> > > > > ---
> > > > > drivers/dma-buf/dma-resv.c | 33 ++++++++++++---------------------
> > > > > 1 file changed, 12 insertions(+), 21 deletions(-)
> > > > >
> > > > > diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
> > > > > index f26c71747d43..c66bfdde9454 100644
> > > > > --- a/drivers/dma-buf/dma-resv.c
> > > > > +++ b/drivers/dma-buf/dma-resv.c
> > > > > @@ -615,25 +615,21 @@ static inline int
> > > > > dma_resv_test_signaled_single(struct dma_fence *passed_fence)
> > > > > */
> > > > > bool dma_resv_test_signaled(struct dma_resv *obj, bool test_all)
> > > > > {
> > > > > - unsigned int seq, shared_count;
> > > > > + struct dma_fence *fence;
> > > > > + unsigned int seq;
> > > > > int ret;
> > > > > rcu_read_lock();
> > > > > retry:
> > > > > ret = true;
> > > > > - shared_count = 0;
> > > > > seq = read_seqcount_begin(&obj->seq);
> > > > > if (test_all) {
> > > > > struct dma_resv_list *fobj = dma_resv_shared_list(obj);
> > > > > - unsigned int i;
> > > > > -
> > > > > - if (fobj)
> > > > > - shared_count = fobj->shared_count;
> > > > > + unsigned int i, shared_count;
> > > > > + shared_count = fobj ? fobj->shared_count : 0;
> > > > > for (i = 0; i < shared_count; ++i) {
> > > > > - struct dma_fence *fence;
> > > > > -
> > > > > fence = rcu_dereference(fobj->shared[i]);
> > > > > ret = dma_resv_test_signaled_single(fence);
> > > > > if (ret < 0)
> > > > > @@ -641,24 +637,19 @@ bool dma_resv_test_signaled(struct dma_resv
> > > > > *obj, bool test_all)
> > > > > else if (!ret)
> > > > > break;
> > > > > }
> > > > > -
> > > > > - if (read_seqcount_retry(&obj->seq, seq))
> > > > > - goto retry;
> > > > > }
> > > > > - if (!shared_count) {
> > > > > - struct dma_fence *fence_excl = dma_resv_excl_fence(obj);
> > > > > -
> > > > > - if (fence_excl) {
> > > > > - ret = dma_resv_test_signaled_single(fence_excl);
> > > > > - if (ret < 0)
> > > > > - goto retry;
> > > > > + fence = dma_resv_excl_fence(obj);
> > > > > + if (fence) {
> > > > > + ret = dma_resv_test_signaled_single(fence);
> > > > > + if (ret < 0)
> > > > > + goto retry;
> > > > > - if (read_seqcount_retry(&obj->seq, seq))
> > > > > - goto retry;
> > > > > - }
> > > > > }
> > > > > + if (read_seqcount_retry(&obj->seq, seq))
> > > > > + goto retry;
> > > > > +
> > > > > rcu_read_unlock();
> > > > > return ret;
> > > > > }
> > > > > --
> > > > > 2.25.1
> > > > >
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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