Re: [PATCH 2/4 v0.5] sched/umcg: RFC: add userspace atomic helpers

On Thu, Sep 9, 2021 at 1:38 AM Jann Horn [off-list ref] wrote:

On Wed, Sep 8, 2021 at 8:49 PM Peter Oskolkov [off-list ref] wrote:

quoted

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.

[...]

You of course won't be able to simply traverse userspace pointers in
such a situation, only access the specific userspace object that
you've prepared beforehand, but luckily it looks like:

 * idle_server_tid_ptr is only accessed in get_idle_server()
   -> which is used from process_waking_worker()
     -> which runs in sleepable context
 * idle_workers_ptr is accessed from:
   -> enqueue_idle_worker
     -> which is also used from process_waking_worker()

Ah, I guess I got that wrong: process_waking_worker() is sleepable,
but it might be holding the mmap lock, so it can't fault, right? Which
means this would actually be problematic...