Re: [RFC PATCH 2/3 v0.2] sched/umcg: RFC: add userspace atomic helpers

[Jann Horn <jannh@google.com>]

On Thu, Jul 8, 2021 at 9:46 PM Peter Oskolkov <posk@posk.io> wrote:
> Add helper functions to work atomically with userspace 32/64 bit values -
> there are some .*futex.* named helpers, but they are not exactly
> what is needed for UMCG; I haven't found what else I could use, so I
> rolled these.
>
> At the moment only X86_64 is supported.
>
> Note: the helpers should probably go into arch/ somewhere; I have
> them in kernel/sched/umcg.h temporarily for convenience. Please
> let me know where I should put them and how to name them.

Instead of open-coding spinlocks in userspace memory like this (which
some of the reviewers will probably dislike because it will have
issues around priority inversion and such), I wonder whether you could
use an actual futex as your underlying locking primitive?

The most straightforward way to do that would probably be to make the
head structure in userspace look roughly like this?

struct umcg_head {
  u64 head_ptr;
  u32 lock;
};

and then from kernel code, you could build a fastpath that directly
calls cmpxchg_futex_value_locked() and build a fallback based on
do_futex(), or something like that.

There is precedent for using futex from inside the kernel to
communicate with userspace: See mm_release(), which calls do_futex()
with FUTEX_WAKE for the clear_child_tid feature.