On Thu, Feb 06, 2014 at 11:25:46AM +0000, Srivatsa S. Bhat wrote: > Hi Will,
Hello, > On 02/06/2014 04:27 PM, Will Deacon wrote: > > On Wed, Feb 05, 2014 at 10:06:04PM +0000, Srivatsa S. Bhat wrote: > >> Subsystems that want to register CPU hotplug callbacks, as well as perform > >> initialization for the CPUs that are already online, often do it as shown > >> below: > >> > >> get_online_cpus(); > >> > >> for_each_online_cpu(cpu) > >> init_cpu(cpu); > >> > >> register_cpu_notifier(&foobar_cpu_notifier); > >> > >> put_online_cpus(); > >> > >> This is wrong, since it is prone to ABBA deadlocks involving the > >> cpu_add_remove_lock and the cpu_hotplug.lock (when running concurrently > >> with CPU hotplug operations). > > > > Hmm, the code in question (for this patch) runs from an arch_initcall. How > > can you generate CPU hotplug operations at that stage? > > > > You are right - in today's design of the init sequence, CPU hotplug > operations can't be triggered at that time during boot. Phew, so we don't have a bug as the code stands today. > However, there have been proposals to boot CPUs in parallel along with the > rest of the kernel initialization [1] (and that would need full > synchronization > with CPU hotplug even at the initcall stage). Of course this needs a lot of > auditing and modifications to the CPU hotplug notifiers of various subsystems > to make them robust enough to handle the parallel boot; so its not going to > happen very soon. But I felt that it would be a good idea to ensure that we > get the locking/synchronization right, even if the registrations happen very > early during boot today.. you know, just to be on the safer side and also to > make the job easier for whoever that is, who tries to implement parallel > CPU booting again in the future ;-) > > [1]. http://thread.gmane.org/gmane.linux.kernel/1246209 Makes sense, and this seems like a good start. > > so it's best if you take this all via your tree. > > > > Hmm.. I'm not a maintainer myself, so I'm hoping that either Oleg or Rusty > or any of the other CPU hotplug maintainers (Thomas/Peter/Ingo) would be > willing to take these patches through their tree. Well, you can have my ack for this patch: Acked-by: Will Deacon <will.dea...@arm.com> Cheers, Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
