Re: [PATCH tip/core/rcu 06/17] rcu: Eliminate read-modify-write ACCESS_ONCE() calls

[Pranith Kumar <bobby.prani@gmail.com>]

On Tue, Jul 8, 2014 at 4:35 PM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
>
> Good catch, I clearly didn't include enough patterns in my search.
>
> But please see below.  And please rebase onto branch rcu/dev in
> git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git,
> as this patch set does not apply.

OK, I will resend the patch. One question below:

>
>                                                         Thanx, Paul
>
>> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
>> ---
>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> index dac6d20..f500395 100644
>> --- a/kernel/rcu/tree.c
>> +++ b/kernel/rcu/tree.c
>> @@ -1700,7 +1700,7 @@ static int rcu_gp_fqs(struct rcu_state *rsp, int
>> fqs_state_in)
>>         if (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) {
>>                 raw_spin_lock_irq(&rnp->lock);
>>                 smp_mb__after_unlock_lock();
>> -               ACCESS_ONCE(rsp->gp_flags) &= ~RCU_GP_FLAG_FQS;
>> +               ACCESS_ONCE(rsp->gp_flags) = rsp->gp_flags & ~RCU_GP_FLAG_FQS;
>
> Here we need ACCESS_ONCE() around both instances of rsp->gp_flags.

I see that all accesses of gp_flags are wrapped with ACCESS_ONCE(). Is
there any reason why we can't declare it as 'volatile' and not use
ACCESS_ONCE everywhere?

-- 
Pranith