Messages in this thread |  | | | Date | Thu, 18 Oct 2007 14:29:59 +0530 | | From | Gautham R Shenoy <> | | Subject | Re: [RFC PATCH 2/4] Rename lock_cpu_hotplug to get_online_cpus |
On Thu, Oct 18, 2007 at 03:22:21AM -0500, Nathan Lynch wrote: > Gautham R Shenoy wrote: > > Hi Nathan, > > > Hi Gautham- > > > > > > Gautham R Shenoy wrote: > > > > Replace all lock_cpu_hotplug/unlock_cpu_hotplug from the kernel and use > > > > get_online_cpus and put_online_cpus instead as it highlights > > > > the refcount semantics in these operations. > > > > > > Something other than "get_online_cpus", please? lock_cpu_hotplug() > > > protects cpu_present_map as well as cpu_online_map. For example, some > > > of the powerpc code modified in this patch is made a bit less clear > > > because it is manipulating cpu_present_map, not cpu_online_map. > > > > A quick look at the code, and I am wondering why is lock_cpu_hotplug() > > used there in the first place. It doesn't look like we require any > > protection against cpus coming up/ going down in the code below, > > since the cpu-hotplug operation doesn't affect the cpu_present_map. > > The locking is necessary. Changes to cpu_online_map and > cpu_present_map must be serialized; otherwise you could end up trying > to online a cpu as it is being removed (i.e. cleared from > cpu_present_map). Online operations must check that a cpu is present > before bringing it up (kernel/cpu.c):
Fair enough!
But we are not protecting the cpu_present_map here using lock_cpu_hotplug(), now are we?
The lock_cpu_hotplug() here, ensures that no cpu-hotplug operations occur in parallel with a processor add or a processor remove. IOW, we're still ensuring that the cpu_online_map doesn't change while we're changing the cpu_present_map. So I don't see why the name get_online_cpus() should be a problem here. May be we could add a comment as to why we don't want a cpu-hotplug operation to happen while we're adding/removing a processor.
Unless of course, lock_cpu_hotplug() is also used to serialize the add_processor and remove_processor operations. Is that the case here ?
Thanks and Regards gautham. > > /* Requires cpu_add_remove_lock to be held */ > static int __cpuinit _cpu_up(unsigned int cpu, int tasks_frozen) > { > int ret, nr_calls = 0; > void *hcpu = (void *)(long)cpu; > unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0; > > if (cpu_online(cpu) || !cpu_present(cpu)) > return -EINVAL; > .... > > > Can't we use another mutex here instead of the cpu_hotplug mutex here ? > > I guess so, but I don't really see the need... >
-- Gautham R Shenoy Linux Technology Center IBM India. "Freedom comes with a price tag of responsibility, which is still a bargain, because Freedom is priceless!" - 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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