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

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

On Mon, 15 Aug 2022 at 07:34, Yonghong Song <yhs@fb.com> wrote:
>
> 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:
> [...]
> >
> > 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.
>

Yep, I actually wrote a prototype list implementation (it's very close
so I can probably post it very soon as an RFC, at this point) which is
using declaration tags like this. For now only one bpf_spin_lock is
there in a map value or globally, so I didn't add guarded_by, but if
you look in [0] it can be used to tag e.g. value_type of a specific
bpf_list_head, which node in that value type can be linked, etc.

  [0]: https://github.com/kkdwivedi/linux/commits/bpf-list