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

["Xu, Yanfei" <yanfei.xu@xxxxxxxxxxxxx>]

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