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

[Gautham R Shenoy <ego@in.ibm.com>]

Hi Nathan, 

> Gautham R Shenoy wrote:
> > 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?
> 
> Yes, we are.  In addition to the above, updates to cpu_present_map
> have to be serialized.  pseries_add_processor can be summed up as
> "find the first N unset bits in cpu_present_map and set them".  That's
> not an atomic operation, so some kind of mutual exclusion is needed.
> 

Okay. But other than pseries_add_processor and pseries_remove_processor,
are there any other places where we _change_ the cpu_present_map ?

I agree that we need some kind of synchronization for threads which read
the cpu_present_map. But probably we can use a seperate mutex for that.



> 
> > The lock_cpu_hotplug() here, ensures that no cpu-hotplug operations
> > occur in parallel with a processor add or a processor remove. 
> 
> That's one important effect, but not the only one (see above).
> 
> 
> > 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.
> 
> The naming is a problem IMO for two reasons:
> 
> - lock_cpu_hotplug() protects cpu_present_map as well as
>   cpu_online_map (sigh, I see that Documentation/cpu-hotplug.txt
>   disagrees with me, but my statement holds for powerpc, at least).
> 
> - get_online_cpus() implies reference count semantics (as stated in
>   the changelog) but AFAICT it really has a reference count
>   implementation with read-write locking semantics.
> 
> Hmm, I think there's another problem here.  With your changes, code
> which relies on the mutual exclusion behavior of lock_cpu_hotplug()
> (such as pseries_add/remove_processor) will now be able to run
> concurrently.  Probably those functions should use
> cpu_hotplug_begin/end instead.

One of the primary reasons to move away from the mutex semantics for
cpu-hotplug protection, was that there were a lot of places where
lock_cpu_hotplug() was used for protecting local data structures too,
when they had nothing to do with cpu-hotplug as such, and it resulted 
in a whole mess. It took people quite sometime to sort things out 
with the cpufreq subsystem.

Probably it would be a lot cleaner if we use get_online_cpus() for
protection against cpu-hotplug and use specific mutexes for serializing
accesses to local data structures. Thoughts?

Thanks and Regards
gautham.

-- 
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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