Re: [PATCH -V3 -mm] mm, swap: Fix race between swapoff and some swap operations

["Huang\, Ying" <ying.huang@intel.com>]

"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> writes:

> On Mon, Dec 18, 2017 at 03:41:41PM +0800, Huang, Ying wrote:
>> "Huang, Ying" <ying.huang@intel.com> writes:
>> And, it appears that if we replace smp_wmb() in _enable_swap_info() with
>> stop_machine() in some way, we can avoid smp_rmb() in get_swap_device().
>> This can reduce overhead in normal path further.  Can we get same effect
>> with RCU?  For example, use synchronize_rcu() instead of stop_machine()?
>> 
>> Hi, Paul, can you help me on this?
>
> If the key loads before and after the smp_rmb() are within the same
> RCU read-side critical section, -and- if one of the critical writes is
> before the synchronize_rcu() and the other critical write is after the
> synchronize_rcu(), then you normally don't need the smp_rmb().
>
> Otherwise, you likely do still need the smp_rmb().

My question may be too general, let make it more specific.  For the
following program,

"
int a;
int b;

void intialize(void)
{
        a = 1;
        synchronize_rcu();
        b = 2;
}

void test(void)
{
        int c;

        rcu_read_lock();
        c = b;
        /* ignored smp_rmb() */
        if (c)
                pr_info("a=%d\n", a);
        rcu_read_unlock();
}
"

Is it possible for it to show

"
a=0
"

in kernel log?


If it couldn't, this could be a useful usage model of RCU to accelerate
hot path.

Best Regards,
Huang, Ying

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