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

[Yonghong Song <yhs@fb.com>]

On 8/10/22 2:46 PM, Kumar Kartikeya Dwivedi wrote:
> On Tue, 2 Aug 2022 at 00:23, Alexei Starovoitov <ast@fb.com> wrote:
>>
>> 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.
> 
> Just to continue brainstorming: Comments on this?
> 
> Instead of a rbtree map, you have a struct bpf_rbtree global variable
> which works like a rbtree. To associate a lock with multiple
> bpf_rbtree, you do clang style thread safety annotation in the bpf
> program:
> 
> #define __guarded_by(lock) __attribute__((btf_type_tag("guarded_by:" #lock))
> 
> struct bpf_spin_lock shared_lock;
> struct bpf_rbtree rbtree1 __guarded_by(shared_lock);
> struct bpf_rbtree rbtree2 __guarded_by(shared_lock);

For the above __guarded_by macro, we should use
btf_decl_tag instead of btf_type_tag

#define __guarded_by(lock) __attribute__((btf_decl_tag("guarded_by:" #lock))

Currently, in llvm implementation, btf_type_tag only applies
to pointee type's. btf_decl_tag can apply to global variable,
function argument, function return value and struct/union members.
So btf_decl_tag shoul work for the above global variable case or
below struct rbtree_set member case.

> 
> guarded_by tag is mandatory for the rbtree. Verifier ensures
> shared_lock spin lock is held whenever rbtree1 or rbtree2 is being
> accessed, and whitelists add/remove helpers inside the critical
> section.
> 
> I don't think associating locks to rbtree dynamically is a hard
> requirement for your use case? But if you need that, you may probably
> also allocate sets of rbtree that are part of the same lock "class"
> dynamically using bpf_kptr_alloc, and do similar annotation for the
> struct being allocated.
> struct rbtree_set {
>    struct bpf_spin_lock lock;
>    struct bpf_rbtree rbtree1 __guarded_by(lock);
>    struct bpf_rbtree rbtree2 __guarded_by(lock);
> };
> struct rbtree_set *s = bpf_kptr_alloc(sizeof(*s), btf_local_type_id(*s));
> // Just stash the pointer somewhere with kptr_xchg
> On bpf_kptr_free, the verifier knows this is not a "trivial" struct,
> so inserts destructor calls for bpf_rbtree fields during program
> fixup.
> 
> The main insight is that lock and rbtree are part of the same
> allocation (map value for global case, ptr_to_btf_id for dynamic case)
> so the locked state can be bound to the reg state in the verifier.
> Then we can also make this new rbtree API use kfuncs instead of UAPI
> helpers, to get some field experience before baking it in.
> 
> Any opinions? Any brainos or deficiencies in the scheme above?
> 
> Background: I have been thinking about how I can bind kptr and normal
> data synchronization without having unneeded atomic xchg cost when
> lock is already protecting kptr. In my tests this guarded_by
> annotation has been proving very useful (you mark data and kptr
> protected by lock as guarded_by some spin lock member in same map
> value, verifier ensures lock is held during access, and kptr_xchg for
> guarded kptr is lowered to non-atomic load/store assuming no
> concurrent access, and kptr_xchg from outside the lock section is
> rejected).