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Subject: Re: [PATCH 1/2] rcu: Delete a redundant check in check_cpu_stall()
To:     paulmck@kernel.org
Cc:     Frederic Weisbecker <frederic@kernel.org>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Zqiang <qiang.zhang1211@gmail.com>, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, Zhen Lei <thunder.leizhen@huawei.com>
References: <20230721075716.857-1-thunder.leizhen@huaweicloud.com>
 <20230721075716.857-2-thunder.leizhen@huaweicloud.com>
 <11e9e09a-c47f-4690-a7c5-9a08913c3e5d@paulmck-laptop>
From:   "Leizhen (ThunderTown)" <thunder.leizhen@huaweicloud.com>
Message-ID: <042c0f6d-3579-228c-dea4-5f2c880a43f7@huaweicloud.com>
Date:   Mon, 24 Jul 2023 09:38:48 +0800
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On 2023/7/22 6:37, Paul E. McKenney wrote:
> On Fri, Jul 21, 2023 at 03:57:15PM +0800, thunder.leizhen@huaweicloud.com wrote:
>> From: Zhen Lei <thunder.leizhen@huawei.com>
>>
>> j = jiffies;
>> js = READ_ONCE(rcu_state.jiffies_stall);			(1)
>> if (ULONG_CMP_LT(j, js))					(2)
>> 	return;
>>
>> if (cmpxchg(&rcu_state.jiffies_stall, js, jn) == js)		(3)
>> 	didstall = true;
>>
>> if (didstall && READ_ONCE(rcu_state.jiffies_stall) == jn) {	(4)
>> 	jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
>> 	WRITE_ONCE(rcu_state.jiffies_stall, jn);
>> }
>>
>> For ease of description, the pseudo code is extracted as above. First,
>> assume that only one CPU is operating, the condition 'didstall' is true
>> means that (3) succeeds. That is, the value of rcu_state.jiffies_stall
>> must be 'jn'.
>>
>> Then, assuming that another CPU is also operating at the same time, there
>> are two cases:
>> 1. That CPU sees the updated result at (1), it returns at (2).
>> 2. That CPU does not see the updated result at (1), it fails at (3) and
>>    cmpxchg returns jn. So that, didstall cannot be true.
> 
> The history behind this one is that there are races in which the stall
> can end in the midst of check_cpu_stall().  For example, when the activity
> of producing the stall warning breaks things loose.
> 
> And yes, long ago, I figured that if things had been static for so
> many seconds, they were unlikely to change, and thus omitted any and
> all synchronization.  The Linux kernel taught me better.  ;-)

Okay, got it, thank you

> 
> 							Thanx, Paul
> 
>> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
>> ---
>>  kernel/rcu/tree_stall.h | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
>> index cc884cd49e026a3..371713f3f7d15d9 100644
>> --- a/kernel/rcu/tree_stall.h
>> +++ b/kernel/rcu/tree_stall.h
>> @@ -794,7 +794,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
>>  			rcu_ftrace_dump(DUMP_ALL);
>>  		didstall = true;
>>  	}
>> -	if (didstall && READ_ONCE(rcu_state.jiffies_stall) == jn) {
>> +	if (didstall) {
>>  		jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
>>  		WRITE_ONCE(rcu_state.jiffies_stall, jn);
>>  	}
>> -- 
>> 2.25.1
>>
> .
> 

-- 
Regards,
  Zhen Lei