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SubjectRe: [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@windriver.com>
>> CC: Dipankar Sarma <dipankar@in.ibm.com>
>> CC: Paul E. McKenney <paulmck@linux.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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