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

[Srivatsa Vaddagiri <vatsa@in.ibm.com>]

On Fri, Dec 09, 2005 at 10:17:38PM +0300, Oleg Nesterov wrote:
> 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) ?

AFAIK the new value of ->cur should be visible to other CPUs when smp_mb() 
returns. Here's a definition of smp_mb() from Paul's article:

smp_mb(): "memory barrier" that orders both loads and stores. This means loads 
and stores preceding the memory barrier are committed to memory before any 
loads and stores following the memory barrier.

[ http://www.linuxjournal.com/article/8211 ]

> 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?

Again, going by the above article, I would expect CPU1 to see the new value of 
var.


-- 


Thanks and Regards,
Srivatsa Vaddagiri,
Linux Technology Center,
IBM Software Labs,
Bangalore, INDIA - 560017