lkml.org 
[lkml]   [2021]   [Apr]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [Qestion] Is preempt_disable/enable needed in non-preemption code path
On Thu, Apr 15, 2021 at 11:04:05PM +0800, Xu, Yanfei wrote:
> Hi experts,
>
> I am learning rcu mechanism and its codes. When looking at the
> rcu_blocking_is_gp(), I found there is a pair preemption disable/enable
> operation in non-preemption code path. And it has been a long time. I can't
> understand why we need it? Is there some thing I missed? If not, can we
> remove the unnecessary operation like blow?

Good point, you are right that preemption is disabled anyway in that block
of code. However, preempt_disable() and preempt_enable() also prevent the
compiler from moving that READ_ONCE() around. So my question to you is
whether it is safe to remove those statements entirely or whether they
should instead be replaced by barrier() or similar.

Thanx, Paul

> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index da6f5213fb74..c6d95a00715e 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3703,7 +3703,6 @@ static int rcu_blocking_is_gp(void)
> if (IS_ENABLED(CONFIG_PREEMPTION))
> return rcu_scheduler_active == RCU_SCHEDULER_INACTIVE;
> might_sleep(); /* Check for RCU read-side critical section. */
> - preempt_disable();
> /*
> * If the rcu_state.n_online_cpus counter is equal to one,
> * there is only one CPU, and that CPU sees all prior accesses
> @@ -3718,7 +3717,6 @@ static int rcu_blocking_is_gp(void)
> * Those memory barriers are provided by CPU-hotplug code.
> */
> ret = READ_ONCE(rcu_state.n_online_cpus) <= 1;
> - preempt_enable();
> return ret;
> }
>
>
>
> Best regards,
> Yanfei

\
 
 \ /
  Last update: 2021-04-15 17:44    [W:0.060 / U:0.024 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site