Re: [Linaro-mm-sig] [PATCH 01/14] dma-buf: add dma_resv_for_each_fence_unlocked

On Thu, Sep 16, 2021 at 10:50 AM Christian König ckoenig.leichtzumerken@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@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.

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