Messages in this thread | | | Date | Tue, 01 May 2012 19:28:40 +0530 | From | "Srivatsa S. Bhat" <> | Subject | Re: get/put_online_cpus documentation wrong? |
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On 05/01/2012 04:56 PM, Gilad Ben-Yossef wrote:
> On Tue, May 1, 2012 at 3:12 AM, Paul Mackerras <paulus@samba.org> wrote: >> >> In Documentation/cpu-hotplug.txt it says: >> >> " #include <linux/cpu.h> >> get_online_cpus() and put_online_cpus(): >> >> The above calls are used to inhibit cpu hotplug operations. While the >> cpu_hotplug.refcount is non zero, the cpu_online_mask will not change. >> If you merely need to avoid cpus going away, you could also use >> preempt_disable() and preempt_enable() for those sections. >> Just remember the critical section cannot call any >> function that can sleep or schedule this process away." > > > I *think* the critical section remark here is referring to using > preempt_disable/enable(), > rather then the use of get/put_online_cpus(). >
Yes, what Gilad said is right.
Using preempt_disable()/preempt_enable() around a piece of code is a light-weight way of preventing CPUs from going away (CPUs going offline) while executing that code. (However, they don't prevent new CPUs from coming online). So, if you are using preempt_disable|enable() to prevent some CPU from going offline, then the usual rules for using preempt_disable|enable() apply - that code should not sleep.
But if you want to avoid CPU hotplug entirely (both CPU offlining and onlining), then you should use get/put_online_cpus(). You can use this around any piece of code, including those which can sleep.
Regards, Srivatsa S. Bhat IBM Linux Technology Center
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