Re: [RFC PATCH bpf-next 05/11] bpf: Add bpf_spin_lock member to rbtree

[Alexei Starovoitov <ast@fb.com>]

On 7/22/22 11:34 AM, Dave Marchevsky wrote:
> This patch adds a struct bpf_spin_lock *lock member to bpf_rbtree, as
> well as a bpf_rbtree_get_lock helper which allows bpf programs to access
> the lock.
> 
> Ideally the bpf_spin_lock would be created independently oustide of the
> tree and associated with it before the tree is used, either as part of
> map definition or via some call like rbtree_init(&rbtree, &lock). Doing
> this in an ergonomic way is proving harder than expected, so for now use
> this workaround.
> 
> Why is creating the bpf_spin_lock independently and associating it with
> the tree preferable? Because we want to be able to transfer nodes
> between trees atomically, and for this to work need same lock associated
> with 2 trees.

Right. We need one lock to protect multiple rbtrees.
Since add/find/remove helpers will look into rbtree->lock
the two different rbtree (== map) have to point to the same lock.
Other than rbtree_init(&rbtree, &lock) what would be an alternative ?

> 
> Further locking-related patches will make it possible for the lock to be
> used in BPF programs and add code which enforces that the lock is held
> when doing any operation on the tree.
> 
> Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
> ---
>   include/uapi/linux/bpf.h       |  7 +++++++
>   kernel/bpf/helpers.c           |  3 +++
>   kernel/bpf/rbtree.c            | 24 ++++++++++++++++++++++++
>   tools/include/uapi/linux/bpf.h |  7 +++++++
>   4 files changed, 41 insertions(+)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 4688ce88caf4..c677d92de3bc 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -5385,6 +5385,12 @@ union bpf_attr {
>    *	Return
>    *		0
>    *
> + * void *bpf_rbtree_get_lock(struct bpf_map *map)
> + *	Description
> + *		Return the bpf_spin_lock associated with the rbtree
> + *
> + *	Return
> + *		Ptr to lock
>    */
>   #define __BPF_FUNC_MAPPER(FN)		\
>   	FN(unspec),			\
> @@ -5600,6 +5606,7 @@ union bpf_attr {
>   	FN(rbtree_find),		\
>   	FN(rbtree_remove),		\
>   	FN(rbtree_free_node),		\
> +	FN(rbtree_get_lock),		\
>   	/* */
>   
>   /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 35eb66d11bf6..257a808bb767 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -1587,6 +1587,7 @@ const struct bpf_func_proto bpf_rbtree_add_proto __weak;
>   const struct bpf_func_proto bpf_rbtree_find_proto __weak;
>   const struct bpf_func_proto bpf_rbtree_remove_proto __weak;
>   const struct bpf_func_proto bpf_rbtree_free_node_proto __weak;
> +const struct bpf_func_proto bpf_rbtree_get_lock_proto __weak;
>   
>   const struct bpf_func_proto *
>   bpf_base_func_proto(enum bpf_func_id func_id)
> @@ -1686,6 +1687,8 @@ bpf_base_func_proto(enum bpf_func_id func_id)
>   		return &bpf_rbtree_remove_proto;
>   	case BPF_FUNC_rbtree_free_node:
>   		return &bpf_rbtree_free_node_proto;
> +	case BPF_FUNC_rbtree_get_lock:
> +		return &bpf_rbtree_get_lock_proto;
>   	default:
>   		break;
>   	}
> diff --git a/kernel/bpf/rbtree.c b/kernel/bpf/rbtree.c
> index 250d62210804..c6f0a2a083f6 100644
> --- a/kernel/bpf/rbtree.c
> +++ b/kernel/bpf/rbtree.c
> @@ -9,6 +9,7 @@
>   struct bpf_rbtree {
>   	struct bpf_map map;
>   	struct rb_root_cached root;
> +	struct bpf_spin_lock *lock;
>   };
>   
>   BTF_ID_LIST_SINGLE(bpf_rbtree_btf_ids, struct, rb_node);
> @@ -39,6 +40,14 @@ static struct bpf_map *rbtree_map_alloc(union bpf_attr *attr)
>   
>   	tree->root = RB_ROOT_CACHED;
>   	bpf_map_init_from_attr(&tree->map, attr);
> +
> +	tree->lock = bpf_map_kzalloc(&tree->map, sizeof(struct bpf_spin_lock),
> +				     GFP_KERNEL | __GFP_NOWARN);
> +	if (!tree->lock) {
> +		bpf_map_area_free(tree);
> +		return ERR_PTR(-ENOMEM);
> +	}
> +
>   	return &tree->map;
>   }
>   
> @@ -139,6 +148,7 @@ static void rbtree_map_free(struct bpf_map *map)
>   
>   	bpf_rbtree_postorder_for_each_entry_safe(pos, n, &tree->root.rb_root)
>   		kfree(pos);
> +	kfree(tree->lock);
>   	bpf_map_area_free(tree);
>   }
>   
> @@ -238,6 +248,20 @@ static int rbtree_map_get_next_key(struct bpf_map *map, void *key,
>   	return -ENOTSUPP;
>   }
>   
> +BPF_CALL_1(bpf_rbtree_get_lock, struct bpf_map *, map)
> +{
> +	struct bpf_rbtree *tree = container_of(map, struct bpf_rbtree, map);
> +
> +	return (u64)tree->lock;
> +}
> +
> +const struct bpf_func_proto bpf_rbtree_get_lock_proto = {
> +	.func = bpf_rbtree_get_lock,
> +	.gpl_only = true,
> +	.ret_type = RET_PTR_TO_MAP_VALUE,

This hack and

+const struct bpf_func_proto bpf_rbtree_unlock_proto = {
+	.func = bpf_rbtree_unlock,
+	.gpl_only = true,
+	.ret_type = RET_INTEGER,
+	.arg1_type = ARG_PTR_TO_SPIN_LOCK,

may be too much arm twisting to reuse bpf_spin_lock.

May be return ptr_to_btf_id here and bpf_rbtree_lock
should match the type?
It could be new 'struct bpf_lock' in kernel's BTF.

Let's figure out how to associate locks with rbtrees.

Reiterating my proposal that was done earlier in the context
of Delyan's irq_work, but for different type:
How about:
struct bpf_lock *l;
l = bpf_mem_alloc(allocator, bpf_core_type_id_kernel(*l));

that would allocate ptr_to_btf_id object from kernel's btf.
The bpf_lock would have constructor and destructor provided by the 
kernel code.
constructor will set bpf_lock's refcnt to 1.
then bpf_rbtree_init(&map, lock) will bump the refcnt.
and dtor will eventually free it when all rbtrees are freed.
That would be similar to kptr's logic with kptr_get and dtor's 
associated with kernel's btf_id-s.