Re: [RFC PATCH v0.1 4/9] sched/umcg: implement core UMCG API

[Peter Zijlstra <peterz@infradead.org>]

On Fri, May 21, 2021 at 11:33:14PM +0200, Jann Horn wrote:
> >  SYSCALL_DEFINE2(umcg_wake, u32, flags, u32, next_tid)
> >  {
> > -       return -ENOSYS;
> > +       struct umcg_task_data *next_utd;
> > +       struct task_struct *next;
> > +       int ret = -EINVAL;
> > +
> > +       if (!next_tid)
> > +               return -EINVAL;
> > +       if (flags)
> > +               return -EINVAL;
> > +
> > +       next = find_get_task_by_vpid(next_tid);
> > +       if (!next)
> > +               return -ESRCH;
> > +       rcu_read_lock();
> 
> Wouldn't it be more efficient to replace the last 4 lines with the following?
> 
> rcu_read_lock();
> next = find_task_by_vpid(next_tid);
> if (!next) {
>   err = -ESRCH;
>   goto out;
> }

This wakeup crud needs to modify the umcg->state, which is a user
variable. That can't be done under RCU. Weirdly the proposed code
doesn't actually do any of that for undocumented raisins :/

> Then you don't need to use refcounting here...
> 
> > +       next_utd = rcu_dereference(next->umcg_task_data);
> > +       if (!next_utd)
> > +               goto out;
> > +
> > +       if (!READ_ONCE(next_utd->in_wait)) {
> > +               ret = -EAGAIN;
> > +               goto out;
> > +       }
> > +
> > +       ret = wake_up_process(next);
> > +       put_task_struct(next);
> 
> ... and you'd be able to drop this put_task_struct(), too.
> 
> > +       if (ret)
> > +               ret = 0;
> > +       else
> > +               ret = -EAGAIN;
> > +
> > +out:
> > +       rcu_read_unlock();
> > +       return ret;
> >  }
> >
> >  /**
> > @@ -139,5 +325,44 @@ SYSCALL_DEFINE2(umcg_wake, u32, flags, u32, next_tid)
> >  SYSCALL_DEFINE4(umcg_swap, u32, wake_flags, u32, next_tid, u32, wait_flags,
> >                 const struct __kernel_timespec __user *, timeout)
> >  {
> > -       return -ENOSYS;
> > +       struct umcg_task_data *curr_utd;
> > +       struct umcg_task_data *next_utd;
> > +       struct task_struct *next;
> > +       int ret = -EINVAL;
> > +
> > +       rcu_read_lock();
> > +       curr_utd = rcu_dereference(current->umcg_task_data);
> > +
> > +       if (!next_tid || wake_flags || wait_flags || !curr_utd)
> > +               goto out;
> > +
> > +       if (timeout) {
> > +               ret = -EOPNOTSUPP;
> > +               goto out;
> > +       }
> > +
> > +       next = find_get_task_by_vpid(next_tid);
> > +       if (!next) {
> > +               ret = -ESRCH;
> > +               goto out;
> > +       }
> 
> There isn't any type of access check here, right? Any task can wake up
> any other task? That feels a bit weird to me - and if you want to keep
> it as-is, it should probably at least be documented that any task on
> the system can send you spurious wakeups if you opt in to umcg.

You can only send wakeups to other UMCG thingies, per the
next->umcg_task_data check below. That said..

> In contrast, shared futexes can avoid this because they get their
> access control implicitly from the VMA.

Every task must expect spurious wakups at all times, always (for
TASK_NORMAL wakeups that is). There's plenty ways to generate them.

> > +       next_utd = rcu_dereference(next->umcg_task_data);
> > +       if (!next_utd) {
> > +               ret = -EINVAL;
> > +               goto out;
> > +       }