Subject: Re: [RFC patch 14/19] bpf: Use migrate_disable() in hashtab code

From: Mathieu Desnoyers <[email-masked]>
Date: Wed, 19 Feb 2020 10:17:28 -0500 (EST)

----- On Feb 18, 2020, at 6:36 PM, Alexei Starovoitov [email-masked] wrote:

[...]

> If I can use migrate_disable() without RT it will help my work on sleepable
> BPF programs. I would only have to worry about rcu_read_lock() since
> preempt_disable() is nicely addressed.

Hi Alexei,

You may want to consider using SRCU rather than RCU if you need to sleep while
holding a RCU read-side lock.

This is the synchronization approach I consider for adding the ability to take page
faults when doing syscall tracing.

Then you'll be able to replace preempt_disable() by combining SRCU and
migrate_disable():

AFAIU eBPF currently uses preempt_disable() for two reasons:

- Ensure the thread is not migrated,
  -> can be replaced by migrate_disable() in RT
- Provide RCU existence guarantee through sched-RCU
  -> can be replaced by SRCU, which allows sleeping and taking page faults.

I wonder if it would be acceptable to take a page fault while migration is
disabled though ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com