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

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Date:   Fri, 16 Apr 2021 00:18:42 +0800
From:   "Xu, Yanfei" <yanfei.xu@...driver.com>
To:     paulmck@...nel.org
Cc:     rcu@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Qestion] Is preempt_disable/enable needed in non-preemption code
 path



On 4/15/21 11:43 PM, Paul E. 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