On Thursday 14 Jun 2018 at 16:30:37 (+0200), Juri Lelli wrote: > On 14/06/18 15:18, Quentin Perret wrote: > > On Thursday 14 Jun 2018 at 16:11:18 (+0200), Juri Lelli wrote: > > > On 14/06/18 14:58, Quentin Perret wrote: > > > > > > [...] > > > > > > > Hmm not sure if this can help but I think that rebuild_sched_domains() > > > > does _not_ take the hotplug lock before calling partition_sched_domains() > > > > when CONFIG_CPUSETS=n. But it does take it for CONFIG_CPUSETS=y. > > > > > > Did you mean cpuset_mutex? > > > > Nope, I really meant the cpu_hotplug_lock ! > > > > With CONFIG_CPUSETS=n, rebuild_sched_domains() calls > > partition_sched_domains() directly: > > > > https://elixir.bootlin.com/linux/latest/source/include/linux/cpuset.h#L255 > > > > But with CONFIG_CPUSETS=y, rebuild_sched_domains() calls, > > rebuild_sched_domains_locked(), which calls get_online_cpus() which > > calls cpus_read_lock(), which does percpu_down_read(&cpu_hotplug_lock). > > And all that happens before calling partition_sched_domains(). > > Ah, right! > > > So yeah, the point I was trying to make is that there is an inconsistency > > here, maybe for a good reason ? Maybe related to the issue you're seeing ? > > The config that came with the 0day splat was indeed CONFIG_CPUSETS=n. > > So, in this case IIUC we hit the !doms_new branch of partition_sched_ > domains, which uses cpu_active_mask (and cpu_possible_mask indirectly). > Should this be still protected by the hotplug lock then? Hmm I'm not sure ... But looking at your call trace, it seems that the issue happens when sched_cpu_deactivate() is called (not sure why this is called during boot BTW ?), which calls cpuset_update_active_cpus(). And again, for CONFIG_CPUSETS=n, that defaults to a raw call to partition_sched_domain(), but with ndoms_new=1, and no lock taken. I'm still not sure if this is done like that for a good reason, or if this is actually an issue that this patch caught nicely ... Quentin -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
