linux-kernel - Re: [RFC PATCH 2/4] Rename lock_cpu_hotplug to get_online

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Date:	Thu, 18 Oct 2007 14:29:59 +0530
From:	Gautham R Shenoy <ego@...ibm.com>
To:	Nathan Lynch <ntl@...ox.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Rusty Russel <rusty@...tcorp.com.au>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Oleg Nesterov <oleg@...sign.ru>, Ingo Molnar <mingo@...e.hu>,
	Paul E McKenney <paulmck@...ibm.com>
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!"
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