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. > 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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
