Re: [RFC PATCH 2/4] Rename lock_cpu_hotplug to get_online_cpus

[Nathan Lynch <ntl@pobox.com>]

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):

/* 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...

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