From: "Paul E. McKenney" <paulmck@kernel.org>
To: "Xu, Yanfei" <yanfei.xu@windriver.com>
Cc: rcu@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [Qestion] Is preempt_disable/enable needed in non-preemption code path
Date: Thu, 15 Apr 2021 08:43:26 -0700 [thread overview]
Message-ID: <20210415154326.GF4510@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <b068d707-0ac7-4840-a297-112731972d8a@windriver.com>
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
next prev parent reply other threads:[~2021-04-15 15:43 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-15 15:04 [Qestion] Is preempt_disable/enable needed in non-preemption code path Xu, Yanfei
2021-04-15 15:43 ` Paul E. McKenney [this message]
2021-04-15 16:18 ` Xu, Yanfei
2021-04-15 17:01 ` Xu, Yanfei
2021-04-15 17:15 ` Paul E. McKenney
2021-04-15 17:07 ` Paul E. McKenney
2021-04-16 10:51 ` Xu, Yanfei
2021-04-16 17:26 ` Paul E. McKenney
2021-04-19 4:58 ` Xu, Yanfei
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