Messages in this thread | | | Date | Fri, 09 Dec 2005 22:17:38 +0300 | From | Oleg Nesterov <> | Subject | Re: [PATCH] Fix RCU race in access of nohz_cpu_mask |
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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'.
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