Re: [PATCH v4] bpf: Remove in_atomic() from bpf_link_put().

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2023-06-15 09:43:11 [-0700], Paul E. McKenney wrote:
> On Wed, Jun 14, 2023 at 10:34:30AM +0200, Sebastian Andrzej Siewior wrote:
> > bpf_free_inode() is invoked as a RCU callback. Usually RCU callbacks are
> > invoked within softirq context. By setting rcutree.use_softirq=0 boot
> > option the RCU callbacks will be invoked in a per-CPU kthread with
> > bottom halves disabled which implies a RCU read section.
> > 
> > On PREEMPT_RT the context remains fully preemptible. The RCU read
> > section however does not allow schedule() invocation. The latter happens
> > in mutex_lock() performed by bpf_trampoline_unlink_prog() originated
> > from bpf_link_put().
> 
> Just to make sure that I understand, you are proposing that the RCU
> callbacks continue to run with BH disabled, but that BH-disabled regions
> are preemptible in kernels built with CONFIG_PREEMPT_RT=y?
> 
> Or did I miss a turn in there somewhere?

I'm not proposing anything, just stating what we have. On PREEMPT_RT
you are preemptible within the RCU callback but must not invoke
schedule(). Similar to the RCU read section on CONFIG_PREEMPT where you
are preemptible but must not invoke schedule(). 

> 
> 							Thanx, Paul

Sebastian