Re: [Intel-gfx] [PATCH 01/26] dma-buf: add dma_resv_for_each_fence_unlocked v2

Daniel Vetter
2021-09-16T12:15:50Z

On Thu, Sep 16, 2021 at 01:30:17PM +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.
>
> v2: fix accessing the shared fences while they might be freed,
> improve kerneldoc, rename _cursor to _iter, add
> dma_resv_iter_is_exclusive
>
> Signed-off-by: Christian König <christian.koenig@amd.com>

Replied in the other thread with the fully typed out example, this really
needs iter_init/next/end here. Or it's just way too fragile and tricky for
a generic helper that we roll out everywhere.
-Daniel

> ---
> drivers/dma-buf/dma-resv.c | 62 ++++++++++++++++++++++++++++++++++++++
> include/linux/dma-resv.h | 50 ++++++++++++++++++++++++++++++
> 2 files changed, 112 insertions(+)
>
> diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
> index 84fbe60629e3..77083170ec3b 100644
> --- a/drivers/dma-buf/dma-resv.c
> +++ b/drivers/dma-buf/dma-resv.c
> @@ -323,6 +323,68 @@ 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_iter *cursor,
> + bool all_fences, bool first)
> +{
> + struct dma_fence *fence = NULL;
> +
> + first |= read_seqcount_retry(&obj->seq, cursor->seq);
> + 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);
> +
> + 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;
> +
> + 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..1cd686384c71 100644
> --- a/include/linux/dma-resv.h
> +++ b/include/linux/dma-resv.h
> @@ -149,6 +149,53 @@ 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 {
> + /** @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_first: true if this is the first returned fence */
> + bool is_first;
> +};
> +
> +/**
> + * dma_resv_for_each_fence_unlocked - fence iterator
> + * @obj: a dma_resv object pointer
> + * @cursor: a struct dma_resv_iter 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))
> +
> +/**
> + * 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;
> +}
> +
> #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 +413,9 @@ 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_iter *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