linux-kernel - Re: [RFC PATCH] rcu: using ACCESS_ONCE() to observe the jiffies

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Date:	Wed, 29 Sep 2010 14:46:18 +0800
From:	DDD <dongdong.deng@...driver.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	dipankar@...ibm.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] rcu: using ACCESS_ONCE() to observe the jiffies_stall/rnp->qsmask
 value

Paul E. McKenney wrote:
> On Tue, Sep 28, 2010 at 04:32:43PM +0800, Dongdong Deng wrote:
>> Using ACCESS_ONCE() to observe the jiffies_stall/rnp->qsmask value
>> due to the caller didn't hold the root_rcu/rnp node's lock.
> 
> Given that the quantities are only used once, ACCESS_ONCE() is strictly
> speaking not necessary.  I am tempted to queue your patch regardless
> for the documentation benefits.
> 
> Or is this patch necessary to fix a failure that you have been seeing?

Hello Pual,

No, I didn't see a failure about it.
I got this patch from code review.  :-)

Thanks,
Dongdong

> In any case, thank you for taking a look at RCU!
> 
> 							Thanx, Paul
> 
>> Signed-off-by: Dongdong Deng <dongdong.deng@...driver.com>
>> CC: Dipankar Sarma <dipankar@...ibm.com>
>> CC: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>> ---
>>  kernel/rcutree.c |    4 ++--
>>  1 files changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
>> index d5bc439..07a7063 100644
>> --- a/kernel/rcutree.c
>> +++ b/kernel/rcutree.c
>> @@ -532,9 +532,9 @@ static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
>>
>>  	if (rcu_cpu_stall_panicking)
>>  		return;
>> -	delta = jiffies - rsp->jiffies_stall;
>> +	delta = jiffies - ACCESS_ONCE(rsp->jiffies_stall);
>>  	rnp = rdp->mynode;
>> -	if ((rnp->qsmask & rdp->grpmask) && delta >= 0) {
>> +	if ((ACCESS_ONCE(rnp->qsmask) & rdp->grpmask) && delta >= 0) {
>>
>>  		/* We haven't checked in, so go dump stack. */
>>  		print_cpu_stall(rsp);
>> -- 
>> 1.6.0.4
>>
> 


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