Messages in this thread | | | Date | Fri, 19 Oct 2007 10:34:56 +0530 | From | Gautham R Shenoy <> | Subject | Re: [RFC PATCH 2/4] Rename lock_cpu_hotplug to get_online_cpus |
| |
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!" - 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/
| |