Messages in this thread | | | Date | Tue, 11 Dec 2012 18:34:11 +0530 | From | "Srivatsa S. Bhat" <> | Subject | Re: [RFC PATCH v3 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context |
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On 12/10/2012 11:45 PM, Oleg Nesterov wrote: > On 12/10, Srivatsa S. Bhat wrote: >> >> On 12/10/2012 02:27 AM, Oleg Nesterov wrote: >>> However. If this is true, then compared to preempt_disable/stop_machine >>> livelock is possible. Probably this is fine, we have the same problem with >>> get_online_cpus(). But if we can accept this fact I feel we can simmplify >>> this somehow... Can't prove, only feel ;) >> >> Not sure I follow.. > > I meant that write_lock_irqsave(&hotplug_rwlock) in take_cpu_down() > can spin "forever". > > Suppose that reader_acked() == T on every CPU, so that > get_online_cpus_atomic() always takes read_lock(&hotplug_rwlock). > > It is possible that this lock will be never released by readers, > > CPU_0 CPU_1 > > get_online_cpus_atomic() > get_online_cpus_atomic() > put_online_cpus_atomic() > > get_online_cpus_atomic() > put_online_cpus_atomic() > > get_online_cpus_atomic() > put_online_cpus_atomic() > > and so on. >
Right, and we can't do anything about it :( Regards, Srivatsa S. Bhat
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