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

Daniel Vetter
2021-09-17T13:23:33Z

On Fri, Sep 17, 2021 at 02:34:48PM +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, add dma_resv_iter_begin/end
>
> Signed-off-by: Christian König <christian.koenig@amd.com>
> ---
> drivers/dma-buf/dma-resv.c | 61 +++++++++++++++++++++++++++
> include/linux/dma-resv.h | 84 ++++++++++++++++++++++++++++++++++++++
> 2 files changed, 145 insertions(+)
>
> diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
> index 84fbe60629e3..3e77cad2c9d4 100644
> --- a/drivers/dma-buf/dma-resv.c
> +++ b/drivers/dma-buf/dma-resv.c
> @@ -323,6 +323,67 @@ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)
> }
> EXPORT_SYMBOL(dma_resv_add_excl_fence);
>
> +/**
> + * dma_resv_iter_walk_unlocked - walk over fences in a dma_resv obj
> + * @cursor: cursor to record the current position
> + * @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 iterration is started over again.
> + */
> +struct dma_fence *dma_resv_iter_walk_unlocked(struct dma_resv_iter *cursor,

Bit ocd, but I'd still just call that iter_next.

> + bool first)

Hm I'd put all the init code into iter_begin ...

> +{
> + struct dma_resv *obj = cursor->obj;

Aren't we missing rcu_read_lock() around the entire thing here?

> +
> + first |= read_seqcount_retry(&obj->seq, cursor->seq);
> + do {
> + /* Drop the reference from the previous round */
> + dma_fence_put(cursor->fence);
> +
> + cursor->is_first = first;
> + if (first) {
> + cursor->seq = read_seqcount_begin(&obj->seq);
> + cursor->index = -1;
> + cursor->fences = dma_resv_shared_list(obj);

And then also call iter_begin from here. That way we guarantee that
read_seqcount_begin is always called before _retry(). It's not a problem
with the seqcount implementation (I think at least), but it definitely
looks funny.

Calling iter_begin here also makes it clear that we're essentially
restarting.

> +
> + cursor->fence = dma_resv_excl_fence(obj);
> + if (cursor->fence &&
> + test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,

Please use the right dma_fence wrapper here for this and don't look at the
bits/flags outside of dma_fence.[hc] code. I just realized that we don't
have the right amount of barriers in there for the fastpath, i.e. if we
have:

x = 0; /* static initializer */

thread a
x = 1;
dma_fence_signal(fence);

thread b;
if (dma_fence_is_signalled(fence))
printk("%i\n", x);

Then you might actually be able to observe x == 0 in thread b. Which is
not what we want at all.

So no open-coding of dma_fence flag bits code outside of drm_fence.[hc]
please. And yes i915-gem code is unfortunately a disaster.

> + &cursor->fence->flags))
> + cursor->fence = NULL;
> + } else {
> + cursor->fence = NULL;
> + }
> +
> + if (cursor->fence) {
> + cursor->fence = dma_fence_get_rcu(cursor->fence);
> + } else if (cursor->all_fences && cursor->fences) {
> + struct dma_resv_list *fences = cursor->fences;
> +
> + while (++cursor->index < fences->shared_count) {
> + cursor->fence = rcu_dereference(
> + fences->shared[cursor->index]);
> + if (!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
> + &cursor->fence->flags))
> + break;
> + }
> + if (cursor->index < fences->shared_count)
> + cursor->fence =
> + dma_fence_get_rcu(cursor->fence);
> + else
> + cursor->fence = NULL;
> + }

The control flow here is very hairy, but I'm not sure how to best do this.
With my suggestion to move the read_seqcount_begin into iter_begin maybe
something like this:

iter_next()
{
do {
dma_fence_put(cursor->fence)
cursor->fence = NULL;

if (cursor->index == -1) { /* reset by iter_begin()
cursor->fence = get_exclusive();
cusor->index++;
} else {
cursor->fence = shared_fences[++cursor->index]
}

if (!dma_fence_is_signalled(cursor->fence))
continue; /* just grab the next fence. */

cursor->fence = dma_fence_get_rcu(cursor->fence);

if (!cursor->fence || read_seqcount_retry()) {
/* we lost the race, restart completely */
iter_begin(); /* ->fence will be cleaned up at beginning of the loop */
continue;
}

return cursor->fence;
} while (true);
}

Maybe I missed something, but that avoids the duplication of all the
tricky code, i.e. checking for signalling, rcu protected conditional
fence_get, and the retry is also nicely at the end.
> +
> + /* For the eventually next round */
> + first = true;
> + } while (read_seqcount_retry(&obj->seq, cursor->seq));
> +
> + return cursor->fence;
> +}
> +EXPORT_SYMBOL_GPL(dma_resv_iter_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..693d16117153 100644
> --- a/include/linux/dma-resv.h
> +++ b/include/linux/dma-resv.h
> @@ -149,6 +149,90 @@ 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 */

If you go with my suggestion (assuming it works): Please add "-1 indicates
to pick the exclusive fence instead."

> + 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;

I think if we just rely on -1 == exclusive fence/is_first we don't need
this one here?

> +};
> +
> +struct dma_fence *dma_resv_iter_walk_unlocked(struct dma_resv_iter *cursor,
> + bool first);
> +
> +/**
> + * 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 iterator over
> + * @all_fences: If all fences should be returned or just the exclusive one

Please add: "Callers must clean up the iterator with dma_resv_iter_end()."

> + */
> +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.

Please add:

"This function must be called every time dma_resv_iter_begin() was called
to clean up any references."
> + */
> +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_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. The RCU read side lock must be hold when using this, but can
> + * be dropped and re-taken as necessary inside the loop. The cursor needs to be
> + * initialized with dma_resv_iter_begin_unlocked() and cleaned up with

We don't have an _unlocked version?

> + * dma_resv_iter_end_unlocked().
> + */
> +#define dma_resv_for_each_fence_unlocked(cursor, fence) \
> + for (fence = dma_resv_iter_walk_unlocked(cursor, true); \
> + fence; fence = dma_resv_iter_walk_unlocked(cursor, false))
> +
> #define dma_resv_held(obj) lockdep_is_held(&(obj)->lock.base)
> #define dma_resv_assert_held(obj) lockdep_assert_held(&(obj)->lock.base)
>
> --
> 2.25.1
>

--
Daniel Vetter
Software Engineer, Intel Corporation