Re: [PATCH] Fix RCU race in access of nohz_cpu_mask

[Oleg Nesterov <oleg@tv-sign.ru>]

Srivatsa Vaddagiri wrote:
> 
> On Thu, Dec 08, 2005 at 10:31:06PM +0300, Oleg Nesterov wrote:
> > I can't see how this change can prevent idle cpus to be included in
> > ->cpumask, cpu can add itself to nohz_cpu_mask right after some other
> > cpu started new grace period.
> 
> Yes that can happen, but if they check for rcu_pending right after that
> it will prevent them from going tickless atleast (which will prevent grace
> periods from being unnecessarily extended). Something like below:
> 
>         CPU0                                    CPU1
> 
>         rcp->cur++;     /* New GP */
> 
>         smp_mb();

I think I need some education on memory barriers.

Does this mb() garantees that the new value of ->cur will be visible
on other cpus immediately after smp_mb() (so that rcu_pending() will
notice it) ?

My understanding is that it only garantees that all stores before it
must be visible before any store after mb. (yes, mb implies rmb, but
I think it does not matter if CPU1 adds itself to nonhz mask after
CPU0 reads nohz_cpu_mask). This means that CPU1 can read the stale
value of ->cur. I guess I am wrong, but I can't prove it to myself.

Could you please clarify this?

Even simpler question:

CPU0
	var = 1;
	wmb();

after that CPU1 does rmb().

Does it garantees that CPU1 will see the new value of var?

> Ideally we would have needed a smp_mb() in CPU1 also between setting CPU1
> in nohz_cpu_mask and checking for rcu_pending(), but I guess it is not needed
> in s390 because of its strong ordering?

I don't know, but please note that s390's definition of smp_mb__after_atomic_inc()
is not a 'nop'.

Oleg.
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