Messages in this thread | | | Date | Fri, 26 Aug 2011 15:57:39 +0200 | From | Oleg Nesterov <> | Subject | Re: [kernel.org users] [KORG] Panics on master backend |
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On 08/26, Yong Zhang wrote: > > On Thu, Aug 25, 2011 at 03:54:29PM +0200, Oleg Nesterov wrote: > > > > Of course it is not TASK_RUNNING, but it can be running or not. > > Yup. Before we go beyond ttwu_remote() in ttwu(), 'cpu' is not safe. > For example, wait_event() could be preempted in between. > > But after we go beyond ttwu_remote(), ->pi_lock will stabilize it.
Yes.
> So after we take Oleg's suggestion("task_cpu(p) == smp_processor_id()"), > things we left is just how to account stat correctly.
Imho, we don't really care. This race is very unlikely, and I think that the "wrong" cpu argument in ttwu_stat() is harmless.
My only point was, this "cpu = task_cpu(p)" looks confusing, as if we can trust it below, during the actual wakeup.
> @@ -2696,7 +2697,12 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) > success = 1; /* we're going to change ->state */ > cpu = task_cpu(p); > > - if (p->on_rq && ttwu_remote(p, wake_flags)) > + /* > + * read cpu for another time if ttwu_remote() success, > + * just to prevent task migration in between, otherwise > + * we maybe account stat incorrectly. > + */ > + if (p->on_rq && ttwu_remote(p, wake_flags, &cpu))
I don't think this makes the things better. p->on_rq can be already false or ttwu_remote() can fail, in this case we still use the result of initial task_cpu().
Oleg.
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