On 02/06/2014 05:09 PM, Will Deacon wrote: > 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.
Yes, that's right. > >> 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> > Great! Thanks a lot Will! Regards, Srivatsa S. Bhat -- 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/
