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Thu, 15 Apr 2021 17:01:25 +0000 Received: from BY5PR11MB4241.namprd11.prod.outlook.com ([fe80::551b:e310:2ae4:3011]) by BY5PR11MB4241.namprd11.prod.outlook.com ([fe80::551b:e310:2ae4:3011%6]) with mapi id 15.20.4042.018; Thu, 15 Apr 2021 17:01:25 +0000 Subject: Re: [Qestion] Is preempt_disable/enable needed in non-preemption code path From: "Xu, Yanfei" To: paulmck@kernel.org Cc: rcu@vger.kernel.org, LKML References: <20210415154326.GF4510@paulmck-ThinkPad-P17-Gen-1> Message-ID: Date: Fri, 16 Apr 2021 01:01:17 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-Originating-IP: [60.247.85.82] X-ClientProxiedBy: HK2PR04CA0086.apcprd04.prod.outlook.com (2603:1096:202:15::30) To BY5PR11MB4241.namprd11.prod.outlook.com (2603:10b6:a03:1ca::13) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [128.224.162.160] (60.247.85.82) by HK2PR04CA0086.apcprd04.prod.outlook.com (2603:1096:202:15::30) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4042.16 via Frontend Transport; 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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 Hi Paul, I objdump the function rcu_blocking_is_gp(): after dropping the barrier(): ffffffff81107c50 : ffffffff81107c50: e8 7b 2a f5 ff callq ffffffff8105a6d0 <__fentry__> ffffffff81107c55: 8b 05 41 fe 7c 01 mov 0x17cfe41(%rip),%eax # ffffffff828d7a9c ffffffff81107c5b: 55 push %rbp ffffffff81107c5c: 48 89 e5 mov %rsp,%rbp ffffffff81107c5f: 5d pop %rbp ffffffff81107c60: 83 f8 01 cmp $0x1,%eax ffffffff81107c63: 0f 9e c0 setle %al ffffffff81107c66: 0f b6 c0 movzbl %al,%eax ffffffff81107c69: c3 retq ffffffff81107c6a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1) the original codes: ffffffff81107ba0 : ffffffff81107ba0: e8 2b 2b f5 ff callq ffffffff8105a6d0 <__fentry__> ffffffff81107ba5: 55 push %rbp ffffffff81107ba6: 48 89 e5 mov %rsp,%rbp ffffffff81107ba9: 8b 05 ed fe 7c 01 mov 0x17cfeed(%rip),%eax # ffffffff828d7a9c ffffffff81107baf: 83 f8 01 cmp $0x1,%eax ffffffff81107bb2: 5d pop %rbp ffffffff81107bb3: 0f 9e c0 setle %al ffffffff81107bb6: 0f b6 c0 movzbl %al,%eax ffffffff81107bb9: c3 retq ffffffff81107bba: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1) umm... It did been reordered by compiler after dropping the barrier(), however, I think the result will not be effected. Right? 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