Messages in this thread | | | Date | Sat, 10 Dec 2005 20:49:51 +0530 | From | Srivatsa Vaddagiri <> | Subject | Re: [PATCH] Fix RCU race in access of nohz_cpu_mask |
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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.
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Thanks and Regards, Srivatsa Vaddagiri, Linux Technology Center, IBM Software Labs, Bangalore, INDIA - 560017 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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