Message-ID: <ZP8+/gjvKFBGR/Y4@cassiopeiae>
Date:   Mon, 11 Sep 2023 18:23:26 +0200
From:   Danilo Krummrich <dakr@...hat.com>
To:     Boris Brezillon <boris.brezillon@...labora.com>
Cc:     airlied@...il.com, daniel@...ll.ch, matthew.brost@...el.com,
        thomas.hellstrom@...ux.intel.com, sarah.walker@...tec.com,
        donald.robson@...tec.com, christian.koenig@....com,
        faith.ekstrand@...labora.com, dri-devel@...ts.freedesktop.org,
        nouveau@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH drm-misc-next v3 6/7] drm/gpuvm: generalize
 dma_resv/extobj handling and GEM validation

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@...hat.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
> 

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