Messages in this thread | | | Date | Thu, 2 Nov 2023 18:32:43 +0100 | Subject | Re: [PATCH drm-misc-next v8 09/12] drm/gpuvm: reference count drm_gpuvm structures | From | Danilo Krummrich <> |
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Hi Thomas,
thanks for your timely response on that!
On 11/2/23 18:09, Thomas Hellström wrote: > On Thu, 2023-11-02 at 00:31 +0100, Danilo Krummrich wrote: >> Implement reference counting for struct drm_gpuvm. >> >> Signed-off-by: Danilo Krummrich <dakr@redhat.com> >> --- >> drivers/gpu/drm/drm_gpuvm.c | 44 +++++++++++++++++++----- >> -- >> drivers/gpu/drm/nouveau/nouveau_uvmm.c | 20 +++++++++--- >> include/drm/drm_gpuvm.h | 31 +++++++++++++++++- >> 3 files changed, 78 insertions(+), 17 deletions(-) >> >> diff --git a/drivers/gpu/drm/drm_gpuvm.c >> b/drivers/gpu/drm/drm_gpuvm.c >> index 53e2c406fb04..6a88eafc5229 100644 >> --- a/drivers/gpu/drm/drm_gpuvm.c >> +++ b/drivers/gpu/drm/drm_gpuvm.c >> @@ -746,6 +746,8 @@ drm_gpuvm_init(struct drm_gpuvm *gpuvm, const >> char *name, >> gpuvm->rb.tree = RB_ROOT_CACHED; >> INIT_LIST_HEAD(&gpuvm->rb.list); >> >> + kref_init(&gpuvm->kref); >> + >> gpuvm->name = name ? name : "unknown"; >> gpuvm->flags = flags; >> gpuvm->ops = ops; >> @@ -770,15 +772,8 @@ drm_gpuvm_init(struct drm_gpuvm *gpuvm, const >> char *name, >> } >> EXPORT_SYMBOL_GPL(drm_gpuvm_init); >> >> -/** >> - * drm_gpuvm_destroy() - cleanup a &drm_gpuvm >> - * @gpuvm: pointer to the &drm_gpuvm to clean up >> - * >> - * Note that it is a bug to call this function on a manager that >> still >> - * holds GPU VA mappings. >> - */ >> -void >> -drm_gpuvm_destroy(struct drm_gpuvm *gpuvm) >> +static void >> +drm_gpuvm_fini(struct drm_gpuvm *gpuvm) >> { >> gpuvm->name = NULL; >> >> @@ -790,7 +785,33 @@ drm_gpuvm_destroy(struct drm_gpuvm *gpuvm) >> >> drm_gem_object_put(gpuvm->r_obj); >> } >> -EXPORT_SYMBOL_GPL(drm_gpuvm_destroy); >> + >> +static void >> +drm_gpuvm_free(struct kref *kref) >> +{ >> + struct drm_gpuvm *gpuvm = container_of(kref, struct >> drm_gpuvm, kref); >> + >> + if (drm_WARN_ON(gpuvm->drm, !gpuvm->ops->vm_free)) >> + return; >> + >> + drm_gpuvm_fini(gpuvm); >> + >> + gpuvm->ops->vm_free(gpuvm); >> +} >> + >> +/** >> + * drm_gpuvm_bo_put() - drop a struct drm_gpuvm reference > copy-paste error in function name. > > Also it appears like xe might put a vm from irq context so we should > document the context where this function call is allowable, and if > applicable add a might_sleep().
From GPUVM PoV I don't see why we can't call this from an IRQ context. It depends on the driver callbacks of GPUVM (->vm_free) and the resv GEM's free callback. Both are controlled by the driver. Hence, I don't see the need for a restriction here.
> > If this function needs to sleep we can work around that in Xe by > keeping an xe-private refcount for the xe vm container, but I'd like to > avoid that if possible and piggy-back on the refcount introduced here. > >> + * @gpuvm: the &drm_gpuvm to release the reference of >> + * >> + * This releases a reference to @gpuvm. >> + */ >> +void >> +drm_gpuvm_put(struct drm_gpuvm *gpuvm) >> +{ >> + if (gpuvm) >> + kref_put(&gpuvm->kref, drm_gpuvm_free); >> +} >> +EXPORT_SYMBOL_GPL(drm_gpuvm_put); >> >> static int >> __drm_gpuva_insert(struct drm_gpuvm *gpuvm, >> @@ -843,7 +864,7 @@ drm_gpuva_insert(struct drm_gpuvm *gpuvm, >> if (unlikely(!drm_gpuvm_range_valid(gpuvm, addr, range))) >> return -EINVAL; >> >> - return __drm_gpuva_insert(gpuvm, va); >> + return __drm_gpuva_insert(drm_gpuvm_get(gpuvm), va); > > Here we leak a reference if __drm_gpuva_insert() fails, and IMO the > reference should be taken where the pointer holding the reference is > assigned (in this case in __drm_gpuva_insert()), or document the > reference transfer from the argument close to the assignment.
Ah, good catch. I had it in __drm_gpuva_insert() originally, but that doesn't work, because __drm_gpuva_insert() is used to insert the kernel_alloc_node. And we need to __drm_gpuva_remove() the kernel_alloc_node from drm_gpuvm_fini(), which is called when the reference count is at zero already. In fact, the __* variants are only there to handle the kernel_alloc_node and this one clearly doesn't need reference counting.
> > But since a va itself is not refcounted it clearly can't outlive the > vm, so is a reference really needed here?
Well, technically, it can. It just doesn't make any sense and would be considered to be a bug. The reference count comes in handy to prevent that in the first place.
I'd like to keep the reference count and just fix up the code.
> > I'd suggest using an accessor that instead of using va->vm uses va- >> vm_bo->vm, to avoid needing to worry about the vm->vm refcount > altoghether.
No, I want to keep that optional. Drivers should be able to use GPUVM to track mappings without being required to implement everything else.
I think PowerVR, for instance, currently uses GPUVM only to track mappings without everything else.
- Danilo
> > Thanks, > Thomas >
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