linux-kernel - [Qestion] Is preempt_disable/enable needed in non-preemption code path

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Message-ID: <b068d707-0ac7-4840-a297-112731972d8a@windriver.com>
Date:   Thu, 15 Apr 2021 23:04:05 +0800
From:   "Xu, Yanfei" <yanfei.xu@...driver.com>
To:     paulmck@...nel.org
Cc:     rcu@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: [Qestion] Is preempt_disable/enable needed in non-preemption code
 path

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?


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