linux-kernel - Re: [RFC 2/4] bpf, security: Add Checmate

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Date:	Fri, 5 Aug 2016 00:05:05 -0700
From:	Sargun Dhillon <sargun@...gun.me>
To:	zhuyj <zyjzyj2000@...il.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	alexei.starovoitov@...il.com, daniel@...earbox.net,
	linux-security-module@...r.kernel.org,
	netdev <netdev@...r.kernel.org>
Subject: Re: [RFC 2/4] bpf, security: Add Checmate

On Thu, Aug 04, 2016 at 05:34:32PM +0800, zhuyj wrote:
>  Sure.
> Is it better to add
> #ifndef CONFIG_PREEMPT_RCU ?
> 
> On Thu, Aug 4, 2016 at 4:28 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > Please do not top post
> >
> > On Thu, 2016-08-04 at 16:08 +0800, zhuyj wrote:
> >>  +void register_checmate_prog_ops(void);
> >> maybe it is extern void register_checmate_prog_ops(void);?
> >>
> >> +       preempt_disable();
> >> +       rcu_read_lock();
> >> IMHO, it is not necessary to use the above 2 since rcu_read_lock will
> >> call preempt_disable.
> >
> > You might double check if this claim is true if CONFIG_PREEMPT_RCU=y
> >
> >
> >
Thanks for your feedback zhuyj, Looking at kernel documentation itself, it looks 
like this is the preferred mechanism[1]. Their example:

 1 preempt_disable();
 2 rcu_read_lock();
 3 do_something();
 4 rcu_read_unlock();
 5 preempt_enable();

But, I think you're right. Do you know if there's a great benefit of doing this? 
Or does it make sense to implement a new macro, a la 
rcu_read_lock_and_preent_disable()?

[1] https://www.kernel.org/doc/Documentation/RCU/Design/Requirements/Requirements.html#Disabling Preemption Does Not Block Grace Periods