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McKenney wrote: > [Please note: This e-mail is from an EXTERNAL e-mail address] > > 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. Thanks for your reply! :) Yes, preempt_disable() and preempt_enable() defined in !preemption are barrier(). barrier can prevent from reordering that READ_ONCE(), but base on my current understanding, volatile in READ_ONCE can also tell the compiler not to reorder it. So, I think it's safe? Best regards, Yanfei > > 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