Re: [PATCH bpf-next v2 0/2] Introduce bpf_preempt_{disable,enable}

[Jiri Olsa <olsajiri@gmail.com>]

On Wed, Apr 24, 2024 at 03:13:13AM +0000, Kumar Kartikeya Dwivedi wrote:
> This set introduces two kfuncs, bpf_preempt_disable and
> bpf_preempt_enable, which are wrappers around preempt_disable and
> preempt_enable in the kernel. These functions allow a BPF program to
> have code sections where preemption is disabled. There are multiple use
> cases that are served by such a feature, a few are listed below:
> 
> 1. Writing safe per-CPU alogrithms/data structures that work correctly
>    across different contexts.
> 2. Writing safe per-CPU allocators similar to bpf_memalloc on top of
>    array/arena memory blobs.
> 3. Writing locking algorithms in BPF programs natively.
> 
> Note that local_irq_disable/enable equivalent is also needed for proper
> IRQ context protection, but that is a more involved change and will be
> sent later.
> 
> While bpf_preempt_{disable,enable} is not sufficient for all of these
> usage scenarios on its own, it is still necessary.
> 
> The same effect as these kfuncs can in some sense be already achieved
> using the bpf_spin_lock or rcu_read_lock APIs, therefore from the
> standpoint of kernel functionality exposure in the verifier, this is
> well understood territory.
> 
> Note that these helpers do allow calling kernel helpers and kfuncs from
> within the non-preemptible region (unless sleepable). Otherwise, any
> locks built using the preemption helpers will be as limited as
> existing bpf_spin_lock.
> 
> Nesting is allowed by keeping a counter for tracking remaining enables
> required to be performed. Similar approach can be applied to
> rcu_read_locks in a follow up.
> 
> Changelog
> =========
> v1: https://lore.kernel.org/bpf/20240423061922.2295517-1-memxor@gmail.com
> 
>  * Move kfunc BTF ID declerations above css task kfunc for
>    !CONFIG_CGROUPS config (Alexei)
>  * Add test case for global function call in non-preemptible region
>    (Jiri)

Acked-by: Jiri Olsa <jolsa@kernel.org>

jirka

> 
> Kumar Kartikeya Dwivedi (2):
>   bpf: Introduce bpf_preempt_[disable,enable] kfuncs
>   selftests/bpf: Add tests for preempt kfuncs
> 
>  include/linux/bpf_verifier.h                  |   1 +
>  kernel/bpf/helpers.c                          |  12 ++
>  kernel/bpf/verifier.c                         |  71 ++++++++-
>  .../selftests/bpf/prog_tests/preempt_lock.c   |   9 ++
>  .../selftests/bpf/progs/preempt_lock.c        | 135 ++++++++++++++++++
>  5 files changed, 226 insertions(+), 2 deletions(-)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/preempt_lock.c
>  create mode 100644 tools/testing/selftests/bpf/progs/preempt_lock.c
> 
> 
> base-commit: 6e10b6350a67d398c795ac0b93a7bb7103633fe4
> -- 
> 2.43.0
> 
>