Re: [PATCH drm-misc-next v3 6/7] drm/gpuvm: generalize dma_resv/extobj handling and GEM validation

Danilo Krummrich
2023-09-11T20:47:12Z

On Mon, Sep 11, 2023 at 12:35:26PM +0200, Boris Brezillon wrote:
> Hello Danilo,
>
> On Sat, 9 Sep 2023 17:31:13 +0200
> Danilo Krummrich <dakr@redhat.com> wrote:
>
>
> > @@ -632,6 +661,131 @@
> > * }
> > */
> >
> > +/**
> > + * get_next_vm_bo_from_list() - get the next vm_bo element
> > + * @__gpuvm: The GPU VM
> > + * @__list_name: The name of the list we're iterating on
> > + * @__local_list: A pointer to the local list used to store already iterated items
> > + * @__prev_vm_bo: The previous element we got from drm_gpuvm_get_next_cached_vm_bo()
> > + *
> > + * This helper is here to provide lockless list iteration. Lockless as in, the
> > + * iterator releases the lock immediately after picking the first element from
> > + * the list, so list insertion deletion can happen concurrently.
> > + *
> > + * Elements popped from the original list are kept in a local list, so removal
> > + * and is_empty checks can still happen while we're iterating the list.
> > + */
> > +#define get_next_vm_bo_from_list(__gpuvm, __list_name, __local_list, __prev_vm_bo) \
> > + ({ \
> > + struct drm_gpuvm_bo *__vm_bo; \
> > + \
> > + drm_gpuvm_bo_put(__prev_vm_bo); \
> > + \
> > + spin_lock(&(__gpuvm)->__list_name.lock); \
>
> I'm tempted to add a drm_gpuvm::<list_name>::local_list field, so we
> can catch concurrent iterations with something like:
>
> if (!(__gpuvm)->__list_name.local_list)
> (__gpuvm)->__list_name.local_list = __local_list;
> else
> WARN_ON((__gpuvm)->__list_name.local_list != __local_list);
>
> with (__gpuvm)->__list_name.local_list being restored to NULL
> in restore_vm_bo_list().
>
> > + while (!list_empty(&(__gpuvm)->__list_name.list)) { \
> > + __vm_bo = list_first_entry(&(__gpuvm)->__list_name.list, \
> > + struct drm_gpuvm_bo, \
> > + list.entry.__list_name); \
> > + if (drm_gpuvm_bo_get_unless_zero(__vm_bo)) { \
> > + list_move_tail(&(__vm_bo)->list.entry.__list_name, \
> > + __local_list); \
> > + break; \
> > + } else { \
> > + list_del_init(&(__vm_bo)->list.entry.__list_name); \
> > + __vm_bo = NULL; \
> > + } \
> > + } \
> > + spin_unlock(&(__gpuvm)->__list_name.lock); \
> > + \
> > + __vm_bo; \
> > + })
> > +
> > +/**
> > + * for_each_vm_bo_in_list() - internal vm_bo list iterator
> > + *
> > + * This helper is here to provide lockless list iteration. Lockless as in, the
> > + * iterator releases the lock immediately after picking the first element from the
> > + * list, so list insertion and deletion can happen concurrently.
> > + *
> > + * Typical use:
> > + *
> > + * struct drm_gpuvm_bo *vm_bo;
> > + * LIST_HEAD(my_local_list);
> > + *
> > + * ret = 0;
> > + * drm_gpuvm_for_each_vm_bo(gpuvm, <list_name>, &my_local_list, vm_bo) {
> > + * ret = do_something_with_vm_bo(..., vm_bo);
> > + * if (ret)
> > + * break;
> > + * }
> > + * drm_gpuvm_bo_put(vm_bo);
> > + * drm_gpuvm_restore_vm_bo_list(gpuvm, <list_name>, &my_local_list);
>
> The names in this example and the helper names don't match.
>
> > + *
> > + *
> > + * Only used for internal list iterations, not meant to be exposed to the outside
> > + * world.
> > + */
> > +#define for_each_vm_bo_in_list(__gpuvm, __list_name, __local_list, __vm_bo) \
> > + for (__vm_bo = get_next_vm_bo_from_list(__gpuvm, __list_name, \
> > + __local_list, NULL); \
> > + __vm_bo; \
> > + __vm_bo = get_next_vm_bo_from_list(__gpuvm, __list_name, \
> > + __local_list, __vm_bo)) \
> > +
> > +/**
> > + * restore_vm_bo_list() - move vm_bo elements back to their original list
> > + * @__gpuvm: The GPU VM
> > + * @__list_name: The name of the list we're iterating on
> > + * @__local_list: A pointer to the local list used to store already iterated items
> > + *
> > + * When we're done iterating a vm_bo list, we should call restore_vm_bo_list()
> > + * to restore the original state and let new iterations take place.
> > + */
> > +#define restore_vm_bo_list(__gpuvm, __list_name, __local_list) \
> > + do { \
> > + /* Merge back the two lists, moving local list elements to the \
> > + * head to preserve previous ordering, in case it matters. \
> > + */ \
> > + spin_lock(&(__gpuvm)->__list_name.lock); \
> > + list_splice(__local_list, &(__gpuvm)->__list_name.list); \
> > + spin_unlock(&(__gpuvm)->__list_name.lock); \
> > + } while (0)
> > +/**
> > + * drm_gpuvm_bo_list_add() - insert a vm_bo into the given list
> > + * @__vm_bo: the &drm_gpuvm_bo
> > + * @__list_name: the name of the list to insert into
> > + *
> > + * Inserts the given @__vm_bo into the list specified by @__list_name and
> > + * increases the vm_bo's reference count.
> > + */
> > +#define drm_gpuvm_bo_list_add(__vm_bo, __list_name) \
> > + do { \
> > + spin_lock(&(__vm_bo)->vm->__list_name.lock); \
> > + if (list_empty(&(__vm_bo)->list.entry.__list_name)) \
> > + list_add_tail(&(__vm_bo)->list.entry.__list_name, \
> > + &(__vm_bo)->vm->__list_name.list); \
> > + spin_unlock(&(__vm_bo)->vm->__list_name.lock); \
> > + } while (0)
> > +
> > +/**
> > + * drm_gpuvm_bo_list_del() - remove a vm_bo from the given list
> > + * @__vm_bo: the &drm_gpuvm_bo
> > + * @__list_name: the name of the list to insert into
> > + *
> > + * Removes the given @__vm_bo from the list specified by @__list_name and
> > + * decreases the vm_bo's reference count.
> > + */
> > +#define drm_gpuvm_bo_list_del(__vm_bo, __list_name) \
> > + do { \
> > + spin_lock(&(__vm_bo)->vm->__list_name.lock); \
> > + if (!list_empty(&(__vm_bo)->list.entry.__list_name)) \
> > + list_del_init(&(__vm_bo)->list.entry.__list_name); \
> > + spin_unlock(&(__vm_bo)->vm->__list_name.lock); \
> > + } while (0)
> > +
> > +static int __must_check
> > +drm_gpuvm_bo_get_unless_zero(struct drm_gpuvm_bo *vm_bo);
>
> I see no obvious reason to have a forward declaration for this helper,
> if we decide to keep it, let's at least move the declaration here.
>
>
> > @@ -807,6 +1262,14 @@ drm_gpuvm_bo_destroy(struct kref *kref)
> >
> > drm_gem_gpuva_assert_lock_held(vm_bo->obj);
> >
> > + spin_lock(&gpuvm->extobj.lock);
> > + list_del(&vm_bo->list.entry.extobj);
> > + spin_unlock(&gpuvm->extobj.lock);
> > +
> > + spin_lock(&gpuvm->evict.lock);
> > + list_del(&vm_bo->list.entry.evict);
> > + spin_unlock(&gpuvm->evict.lock);
> > +
> > list_del(&vm_bo->list.entry.gem);
> >
> > drm_gem_object_put(obj);
> > @@ -822,6 +1285,11 @@ drm_gpuvm_bo_destroy(struct kref *kref)
> > * @vm_bo: the &drm_gpuvm_bo to release the reference of
> > *
> > * This releases a reference to @vm_bo.
> > + *
> > + * If the reference count drops to zero, the &gpuvm_bo is destroyed, which
> > + * includes removing it from the GEMs gpuva list. Hence, if a call to this
> > + * function can potentially let the reference count to zero the caller must
> > + * hold the dma-resv or driver specific GEM gpuva lock.
>
> Looks like this should have been part of the previous patch. I hate
> the fact we have to worry about GEM gpuva lock being held when we call
> _put() only if the ref drops to zero though. I think I'd feel more
> comfortable if the function was named differently. Maybe _return() or
> _release() to match the _obtain() function, where the object is inserted
> in the GEM vm_bo list. I would also do the lock_is_held() check
> unconditionally, move the list removal in this function with a del_init(),
> and have a WARN_ON(!list_empty) in vm_bo_destroy().
>

We can't move the list removal to drm_gpuvm_bo_put(), we need to make sure we
can't create duplicate drm_gpuvm_bo structures. Everything else pretty much goes
away with a dedicated GEM gpuva list lock, as I had in my first patch series
when I introduced the GPUVA manager. At that time it wasn't always needed, hence
the optional driver specific lock, however with the VM_BO abstraction it really
makes sense to have a dedicated one.

I agree with the other feedback from this reply and will address it in a V4.

> > */
> > void
> > drm_gpuvm_bo_put(struct drm_gpuvm_bo *vm_bo)
> > @@ -831,6 +1299,12 @@ drm_gpuvm_bo_put(struct drm_gpuvm_bo *vm_bo)
> > }
> > EXPORT_SYMBOL_GPL(drm_gpuvm_bo_put);
> >
> > +static int __must_check
> > +drm_gpuvm_bo_get_unless_zero(struct drm_gpuvm_bo *vm_bo)
> > +{
> > + return kref_get_unless_zero(&vm_bo->kref);
>
> Not convinced this helper is needed. It's only used once, and I
> don't think we'll need it elsewhere.
>
> > +}
> > +
> > static struct drm_gpuvm_bo *
> > __drm_gpuvm_bo_find(struct drm_gpuvm *gpuvm,
> > struct drm_gem_object *obj)
>
>
> Regards,
>
> Boris
>