[PATCH 01/14] dma-buf: add dma_resv_for_each_fence_unlocked
Christian König
ckoenig.leichtzumerken at gmail.com
Thu Sep 16 12:49:26 UTC 2021
Am 16.09.21 um 14:14 schrieb Daniel Vetter:
> On Thu, Sep 16, 2021 at 10:50 AM Christian König <ckoenig.leichtzumerken at gmail.com> 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 at amd.com>
>>>> ---
>>>> 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.
Yeah, that's what I've pondered on as well but I thought that avoiding
the extra get/put dance would be more important to avoid.
Anyway, going to change that to make clear what happens here.
But question is can you go over the patch set and see if we can replace
some more dma_fence_for_each_fence_unlock() with
dma_fence_for_each_fence() because the lock is either held or can be
taken? I would have a much better feeling to avoid the unlocked access
in the first place.
Thanks,
Christian.
>
> 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
>>>>
>
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