[Intel-gfx] [PATCH 01/28] dma-buf: add dma_resv_for_each_fence_unlocked v7

[Tvrtko Ursulin]

On 01/10/2021 11:05, 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.
> 
> v2: fix accessing the shared fences while they might be freed,
>      improve kerneldoc, rename _cursor to _iter, add
>      dma_resv_iter_is_exclusive, add dma_resv_iter_begin/end
> 
> v3: restructor the code, move rcu_read_lock()/unlock() into the
>      iterator, add dma_resv_iter_is_restarted()
> 
> v4: fix NULL deref when no explicit fence exists, drop superflous
>      rcu_read_lock()/unlock() calls.
> 
> v5: fix typos in the documentation
> 
> v6: fix coding error when excl fence is NULL
> 
> v7: one more logic fix
> 
> Signed-off-by: Christian König <christian.koenig at amd.com>
> ---
>   drivers/dma-buf/dma-resv.c | 100 +++++++++++++++++++++++++++++++++++++
>   include/linux/dma-resv.h   |  95 +++++++++++++++++++++++++++++++++++
>   2 files changed, 195 insertions(+)
> 
> diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
> index 84fbe60629e3..3cbcf66a137e 100644
> --- a/drivers/dma-buf/dma-resv.c
> +++ b/drivers/dma-buf/dma-resv.c
> @@ -323,6 +323,106 @@ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)
>   }
>   EXPORT_SYMBOL(dma_resv_add_excl_fence);
>   
> +/**
> + * dma_resv_iter_restart_unlocked - restart the unlocked iterator
> + * @cursor: The dma_resv_iter object to restart
> + *
> + * Restart the unlocked iteration by initializing the cursor object.
> + */
> +static void dma_resv_iter_restart_unlocked(struct dma_resv_iter *cursor)
> +{
> +	cursor->seq = read_seqcount_begin(&cursor->obj->seq);
> +	cursor->index = -1;
> +	if (cursor->all_fences)
> +		cursor->fences = dma_resv_shared_list(cursor->obj);
> +	else
> +		cursor->fences = NULL;
> +	cursor->is_restarted = true;
> +}
> +
> +/**
> + * dma_resv_iter_walk_unlocked - walk over fences in a dma_resv obj
> + * @cursor: cursor to record the current position
> + *
> + * 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 iteration is started over again.
> + */
> +static void dma_resv_iter_walk_unlocked(struct dma_resv_iter *cursor)
> +{
> +	struct dma_resv *obj = cursor->obj;
> +
> +	do {
> +		/* Drop the reference from the previous round */
> +		dma_fence_put(cursor->fence);
> +
> +		if (cursor->index == -1) {
> +			cursor->fence = dma_resv_excl_fence(obj);
> +			cursor->index++;
> +			if (!cursor->fence)
> +				continue;
> +
> +		} else if (!cursor->fences ||
> +			   cursor->index >= cursor->fences->shared_count) {
> +			cursor->fence = NULL;
> +			break;
> +
> +		} else {
> +			struct dma_resv_list *fences = cursor->fences;
> +			unsigned int idx = cursor->index++;
> +
> +			cursor->fence = rcu_dereference(fences->shared[idx]);
> +		}
> +		cursor->fence = dma_fence_get_rcu(cursor->fence);

Worth having an assert dma_fence_get_rcu does not fail here? Not sure 
that I have seen debug build only asserts though on the DRM core side.

On the bike shedding front, would it be clearer if the continue 
condition on signaled fences was standalone, using the continue 
statement? I'd also possibly re-arrange the three if-else blocks so that 
the end of iteration is not sandwiched between blocks handling exclusive 
and shared, and flow tweaked a bit, like:

   struct dma_fence *fence = cursor->fence;
   int index = cursor->index;

   dma_fence_put(fence);
   fence = NULL;

next:
   if (index == -1) {
	/* Try picking the exclusive fence. */
	index++;
	fence = dma_resv_excl_fence(obj);
	if (!fence)
		goto next;
   } else if (cursor->fences && index < cursor->fences->shared_count) {
	  /* Try picking next shared fence. */
	struct dma_resv_list *fences = cursor->fences;

	fence = rcu_dereference(fences->shared[index++]);
   }

   if (fence) {
	  if (dma_fence_is_signaled(fence))
		goto next; /* Skip signaled. */

	fence = dma_fence_get_rcu(fence);
	WARN_ON(!fence);
   }

   cursor->fence = fence;
   cursor->index = index;

(I started with a loop here but ended with goto based flow since it 
ended up more succinct.)

At least if I don't have a handling flaw in there it looks like easier 
to follow flow to me. Plus picking a not signaled fence works without a 
reference FWIW. How does it look to you?

Regards,

Tvrtko

> +		if (!cursor->fence || !dma_fence_is_signaled(cursor->fence))
> +			break;
> +	} while (true);
> +}
> +
> +/**
> + * dma_resv_iter_first_unlocked - first fence in an unlocked dma_resv obj.
> + * @cursor: the cursor with the current position
> + *
> + * Returns the first fence from an unlocked dma_resv obj.
> + */
> +struct dma_fence *dma_resv_iter_first_unlocked(struct dma_resv_iter *cursor)
> +{
> +	rcu_read_lock();
> +	do {
> +		dma_resv_iter_restart_unlocked(cursor);
> +		dma_resv_iter_walk_unlocked(cursor);
> +	} while (read_seqcount_retry(&cursor->obj->seq, cursor->seq));
> +	rcu_read_unlock();
> +
> +	return cursor->fence;
> +}
> +EXPORT_SYMBOL(dma_resv_iter_first_unlocked);
> +
> +/**
> + * dma_resv_iter_next_unlocked - next fence in an unlocked dma_resv obj.
> + * @cursor: the cursor with the current position
> + *
> + * Returns the next fence from an unlocked dma_resv obj.
> + */
> +struct dma_fence *dma_resv_iter_next_unlocked(struct dma_resv_iter *cursor)
> +{
> +	bool restart;
> +
> +	rcu_read_lock();
> +	cursor->is_restarted = false;
> +	restart = read_seqcount_retry(&cursor->obj->seq, cursor->seq);
> +	do {
> +		if (restart)
> +			dma_resv_iter_restart_unlocked(cursor);
> +		dma_resv_iter_walk_unlocked(cursor);
> +		restart = true;
> +	} while (read_seqcount_retry(&cursor->obj->seq, cursor->seq));
> +	rcu_read_unlock();
> +
> +	return cursor->fence;
> +}
> +EXPORT_SYMBOL(dma_resv_iter_next_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..5d7d28cb9008 100644
> --- a/include/linux/dma-resv.h
> +++ b/include/linux/dma-resv.h
> @@ -149,6 +149,101 @@ struct dma_resv {
>   	struct dma_resv_list __rcu *fence;
>   };
>   
> +/**
> + * struct dma_resv_iter - current position into the dma_resv fences
> + *
> + * Don't touch this directly in the driver, use the accessor function instead.
> + */
> +struct dma_resv_iter {
> +	/** @obj: The dma_resv object we iterate over */
> +	struct dma_resv *obj;
> +
> +	/** @all_fences: If all fences should be returned */
> +	bool all_fences;
> +
> +	/** @fence: the currently handled fence */
> +	struct dma_fence *fence;
> +
> +	/** @seq: sequence number to check for modifications */
> +	unsigned int seq;
> +
> +	/** @index: index into the shared fences */
> +	unsigned int index;
> +
> +	/** @fences: the shared fences */
> +	struct dma_resv_list *fences;
> +
> +	/** @is_restarted: true if this is the first returned fence */
> +	bool is_restarted;
> +};
> +
> +struct dma_fence *dma_resv_iter_first_unlocked(struct dma_resv_iter *cursor);
> +struct dma_fence *dma_resv_iter_next_unlocked(struct dma_resv_iter *cursor);
> +
> +/**
> + * dma_resv_iter_begin - initialize a dma_resv_iter object
> + * @cursor: The dma_resv_iter object to initialize
> + * @obj: The dma_resv object which we want to iterate over
> + * @all_fences: If all fences should be returned or just the exclusive one
> + */
> +static inline void dma_resv_iter_begin(struct dma_resv_iter *cursor,
> +				       struct dma_resv *obj,
> +				       bool all_fences)
> +{
> +	cursor->obj = obj;
> +	cursor->all_fences = all_fences;
> +	cursor->fence = NULL;
> +}
> +
> +/**
> + * dma_resv_iter_end - cleanup a dma_resv_iter object
> + * @cursor: the dma_resv_iter object which should be cleaned up
> + *
> + * Make sure that the reference to the fence in the cursor is properly
> + * dropped.
> + */
> +static inline void dma_resv_iter_end(struct dma_resv_iter *cursor)
> +{
> +	dma_fence_put(cursor->fence);
> +}
> +
> +/**
> + * dma_resv_iter_is_exclusive - test if the current fence is the exclusive one
> + * @cursor: the cursor of the current position
> + *
> + * Returns true if the currently returned fence is the exclusive one.
> + */
> +static inline bool dma_resv_iter_is_exclusive(struct dma_resv_iter *cursor)
> +{
> +	return cursor->index == -1;
> +}
> +
> +/**
> + * dma_resv_iter_is_restarted - test if this is the first fence after a restart
> + * @cursor: the cursor with the current position
> + *
> + * Return true if this is the first fence in an iteration after a restart.
> + */
> +static inline bool dma_resv_iter_is_restarted(struct dma_resv_iter *cursor)
> +{
> +	return cursor->is_restarted;
> +}
> +
> +/**
> + * dma_resv_for_each_fence_unlocked - unlocked fence iterator
> + * @cursor: a struct dma_resv_iter pointer
> + * @fence: the current fence
> + *
> + * Iterate over the fences in a struct dma_resv object without holding the
> + * &dma_resv.lock and using RCU instead. The cursor needs to be initialized
> + * with dma_resv_iter_begin() and cleaned up with dma_resv_iter_end(). Inside
> + * the iterator a reference to the dma_fence is held and the RCU lock dropped.
> + * When the dma_resv is modified the iteration starts over again.
> + */
> +#define dma_resv_for_each_fence_unlocked(cursor, fence)			\
> +	for (fence = dma_resv_iter_first_unlocked(cursor);		\
> +	     fence; fence = dma_resv_iter_next_unlocked(cursor))
> +
>   #define dma_resv_held(obj) lockdep_is_held(&(obj)->lock.base)
>   #define dma_resv_assert_held(obj) lockdep_assert_held(&(obj)->lock.base)
>   
>