Am 14.09.21 um 15:07 schrieb Tvrtko Ursulin:
On 14/09/2021 12:25, Christian König wrote:
Am 14.09.21 um 12:53 schrieb Tvrtko Ursulin:
On 13/09/2021 14:16, Christian König wrote:
[SNIP]
+ if (fence) {
+ fence = dma_fence_get_rcu(fence);
+ } else if (all_fences && cursor->fences) {
+ struct dma_resv_list *fences = cursor->fences;
If rcu lock is allowed to be dropped while walking the list what
guarantees list of fences hasn't been freed?
Ah, good point! We need to test the sequence number when we enter the
function as well. Going to fix that.
Right, but just to say, I am still on the fence a bit on the concept
of the unlocked iterator. So for now I am looking only superficially
at the implementation and i915 side of things.
I'm really in favor of taking the lock as well and contain the unlocked
operation into the dma_resv object code and I think Daniel is on
absolutely the same side as well.
But the use cases are as they are for now and I think containing the
internal structure of the dma_resv object is the right next step.
[SNIP]
+/**
+ * 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;
Is_first is useful to callers - like they are legitimately allowed
to look inside this, what could otherwise be private object?
Yes, I was pondering on the same question. Key point is that this is
only used by other dma_resv functions which also use cursor.fences
for example.
So this is only supposed to be used by code working with other
privates of the dma_resv object as well.
Hmmm and you think external callers have no legitimate case of
detecting restarts?
Yes, if somebody needs a snapshot of the current state and can't for
some reason take the lock they should use dma_resv_get_fences() instead.
On the other hand allocating memory in dma_resv_get_fences() has
probably more overhead than just grabbing and releasing the lock.
Or to better say will not have the need of distinguishing between real
restarts and just the first iteration? I need to read more of the
series to get a more complete opinion here.
Yeah, that's indeed a good point. Off hand I don't see any, but we
should probably decide for each place individually if we should take the
lock, allocate memory or use the lockless iterator.
[SNIP]
+};
+
+/**
+ * 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))
Has the fact RCU lock can be dropped so there is potential to walk
over completely different snapshots been discussed?
Well that's basically the heart of the functionality. Even without
dropping the RCU lock there can be an restart at any time when the
dma_resv object is modified.
Hm yes.. that's one of the thing which makes me undecided yet whether
a generalised helper is desirable. For example i915_gem_busy_ioctl, as
converted, is not completely like-for-like. Maybe it is irrelevant for
that one, but then the question needs to be answered for all of the
replacements.
At least if I followed the code correctly - it appears there is
potential the walk restarts from the start (exclusive slot) at any
point during the walk.
Correct, yes.
Because theoretically I think you could take an atomic snapshot of
everything (given you have a cursor object) and then release it on
the end condition.
That's what the dma_resv_get_fences() function is good for, yes. This
one returns an array of fences.
The key difference is that we need to allocate memory for that which
is at least sometimes not feasible or desired.
Ah true.. dma_resv_list is not reference counted to simply grab it
during setup.
Thanks for the review,
Np, it is intriguing to look at the option of code consolidation. Just
need to read more of the series to form a better high level opinion.
Really appreciated, thanks for looking into this.
Regards,
Christian.
Regards,
Tvrtko
Christian.
Regards,
Tvrtko
+
#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);
