On Thu, Sep 16, 2021 at 10:50 AM Christian König <ckoenig.leichtzumerken@xxxxxxxxx> wrote:
> Am 14.09.21 um 19:04 schrieb Daniel Vetter:
> > On Fri, Sep 10, 2021 at 10:26:42AM +0200, Christian König wrote:
> >> Abstract the complexity of iterating over all the fences
> >> in a dma_resv object.
> >>
> >> The new loop handles the whole RCU and retry dance and
> >> returns only fences where we can be sure we grabbed the
> >> right one.
> >>
> >> Signed-off-by: Christian König <christian.koenig@xxxxxxx>
> >> ---
> >> drivers/dma-buf/dma-resv.c | 63 ++++++++++++++++++++++++++++++++++++++
> >> include/linux/dma-resv.h | 36 ++++++++++++++++++++++
> >> 2 files changed, 99 insertions(+)
> >>
> >> diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
> >> index 84fbe60629e3..213a9b7251ca 100644
> >> --- a/drivers/dma-buf/dma-resv.c
> >> +++ b/drivers/dma-buf/dma-resv.c
> >> @@ -323,6 +323,69 @@ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)
> >> }
> >> EXPORT_SYMBOL(dma_resv_add_excl_fence);
> >>
> >> +/**
> >> + * dma_resv_walk_unlocked - walk over fences in a dma_resv obj
> >> + * @obj: the dma_resv object
> >> + * @cursor: cursor to record the current position
> >> + * @all_fences: true returns also the shared fences
> >> + * @first: if we should start over
> >> + *
> >> + * Return all the fences in the dma_resv object which are not yet signaled.
> >> + * The returned fence has an extra local reference so will stay alive.
> >> + * If a concurrent modify is detected the whole iterator is started over again.
> >> + */
> >> +struct dma_fence *dma_resv_walk_unlocked(struct dma_resv *obj,
> >> + struct dma_resv_cursor *cursor,
> >> + bool all_fences, bool first)
> >> +{
> >> + struct dma_fence *fence = NULL;
> >> +
> >> + do {
> >> + /* Drop the reference from the previous round */
> >> + dma_fence_put(fence);
> >> +
> >> + cursor->is_first = first;
> >> + if (first) {
> >> + cursor->seq = read_seqcount_begin(&obj->seq);
> >> + cursor->index = -1;
> >> + cursor->fences = dma_resv_shared_list(obj);
> >> + cursor->is_exclusive = true;
> >> +
> >> + fence = dma_resv_excl_fence(obj);
> >> + if (fence && test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
> >> + &fence->flags))
> >> + fence = NULL;
> >> + } else {
> >> + fence = NULL;
> >> + }
> >> +
> >> + if (fence) {
> >> + fence = dma_fence_get_rcu(fence);
> >> + } else if (all_fences && cursor->fences) {
> >> + struct dma_resv_list *fences = cursor->fences;
> >> +
> >> + cursor->is_exclusive = false;
> >> + while (++cursor->index < fences->shared_count) {
> >> + fence = rcu_dereference(fences->shared[
> >> + cursor->index]);
> >> + if (!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
> >> + &fence->flags))
> >> + break;
> >> + }
> >> + if (cursor->index < fences->shared_count)
> >> + fence = dma_fence_get_rcu(fence);
> >> + else
> >> + fence = NULL;
> >> + }
> >> +
> >> + /* For the eventually next round */
> >> + first = true;
> >> + } while (read_seqcount_retry(&obj->seq, cursor->seq));
> >> +
> >> + return fence;
> >> +}
> >> +EXPORT_SYMBOL_GPL(dma_resv_walk_unlocked);
> >> +
> >> /**
> >> * dma_resv_copy_fences - Copy all fences from src to dst.
> >> * @dst: the destination reservation object
> >> diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
> >> index 9100dd3dc21f..f5b91c292ee0 100644
> >> --- a/include/linux/dma-resv.h
> >> +++ b/include/linux/dma-resv.h
> >> @@ -149,6 +149,39 @@ struct dma_resv {
> >> struct dma_resv_list __rcu *fence;
> >> };
> >>
> >> +/**
> >> + * struct dma_resv_cursor - current position into the dma_resv fences
> >> + * @seq: sequence number to check
> >> + * @index: index into the shared fences
> >> + * @shared: the shared fences
> >> + * @is_first: true if this is the first returned fence
> >> + * @is_exclusive: if the current fence is the exclusive one
> >> + */
> >> +struct dma_resv_cursor {
> >> + unsigned int seq;
> >> + unsigned int index;
> >> + struct dma_resv_list *fences;
> >> + bool is_first;
> >> + bool is_exclusive;
> >> +};
> > A bit a bikeshed, but I think I'd be nice to align this with the other
> > iterators we have, e.g. for the drm_connector list.
> >
> > So struct dma_resv_fence_iter, dma_resv_fence_iter_begin/next/end().
>
> I've renamed the structure to dma_resv_iter.
>
> > Also I think the for_each macro must not include begin/end calls. If we
> > include that then it saves 2 lines of code at the cost of a pile of
> > awkward bugs because people break; out of the loop or return early (only
> > continue is safe) and we leak a fence. Or worse.
> >
> > Explicit begin/end is much more robust at a very marginal cost imo.
>
> The key point is that this makes it quite a bunch more complicated to
> implement. See those functions are easiest when you centralize them and
> try to not spread the functionality into begin/end.
>
> The only thing I could see in the end function would be to drop the
> reference for the dma_fence and that is not really something I would
> like to do because we actually need to keep that reference in a bunch of
> cases.
Yeah but it's extremely fragile. See with drm_connector_iter we also have
the need to grab a reference to that connector in a few place, and I do
think that open-code that is much clearer instead of inheriting a
reference that the for_each macro acquired for you, and which you cleverly
leaked through a break; Compare
for_each_fence(fence) {
if (fence) {
found_fence = fence;
break;
}
}
/* do some itneresting stuff with found_fence */
dma_fence_put(found_fence); /* wtf, where is this fence reference from */
Versus what I'm proposing:
fence_iter_init(&fence_iter)
for_each_fence(fence, &fence_iter) {
if (fence) {
found_fence = fence;
dma_fence_get(found_fence);
break;
}
}
fence_iter_end(&fence_iter)
/* do some itneresting stuff with found_fence */
dma_fence_put(found_fence); /* 100% clear which reference we're putting here */
One of these patterns is maintainable and clear, at the cost of 3 more
lines. The other one is frankly just clever but fragile nonsense.
So yeah I really think we need the iter_init/end/next triple of functions
here. Too clever is no good at all. And yes that version means you have an
additional kref_get/put in there for the found fence, but I really don't
think that matters in any of these paths here.
Cheers, Daniel
>
> Regards,
> Christian.
>
> >
> > Otherwise I think this fence iterator is a solid concept that yeah we
> > should roll out everywhere.
> > -Daniel
> >
> >> +
> >> +/**
> >> + * dma_resv_for_each_fence_unlocked - fence iterator
> >> + * @obj: a dma_resv object pointer
> >> + * @cursor: a struct dma_resv_cursor pointer
> >> + * @all_fences: true if all fences should be returned
> >> + * @fence: the current fence
> >> + *
> >> + * Iterate over the fences in a struct dma_resv object without holding the
> >> + * dma_resv::lock. The RCU read side lock must be hold when using this, but can
> >> + * be dropped and re-taken as necessary inside the loop. @all_fences controls
> >> + * if the shared fences are returned as well.
> >> + */
> >> +#define dma_resv_for_each_fence_unlocked(obj, cursor, all_fences, fence) \
> >> + for (fence = dma_resv_walk_unlocked(obj, cursor, all_fences, true); \
> >> + fence; dma_fence_put(fence), \
> >> + fence = dma_resv_walk_unlocked(obj, cursor, all_fences, false))
> >> +
> >> #define dma_resv_held(obj) lockdep_is_held(&(obj)->lock.base)
> >> #define dma_resv_assert_held(obj) lockdep_assert_held(&(obj)->lock.base)
> >>
> >> @@ -366,6 +399,9 @@ void dma_resv_fini(struct dma_resv *obj);
> >> int dma_resv_reserve_shared(struct dma_resv *obj, unsigned int num_fences);
> >> void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence);
> >> void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence);
> >> +struct dma_fence *dma_resv_walk_unlocked(struct dma_resv *obj,
> >> + struct dma_resv_cursor *cursor,
> >> + bool first, bool all_fences);
> >> int dma_resv_get_fences(struct dma_resv *obj, struct dma_fence **pfence_excl,
> >> unsigned *pshared_count, struct dma_fence ***pshared);
> >> int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src);
> >> --
> >> 2.25.1
> >>
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
