Messages in this thread | | | Date | Wed, 03 Oct 2012 15:28:17 +0530 | From | "Srivatsa S. Bhat" <> | Subject | Re: Lockdep complains about commit 1331e7a1bb ("rcu: Remove _rcu_barrier() dependency on __stop_machine()") |
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On 10/03/2012 02:54 PM, Jiri Kosina wrote: > On Wed, 3 Oct 2012, Srivatsa S. Bhat wrote: > >>>> static void cpu_hotplug_begin(void) >>>> { >>>> cpu_hotplug.active_writer = current; >>>> >>>> for (;;) { >>>> mutex_lock(&cpu_hotplug.lock); >>>> if (likely(!cpu_hotplug.refcount)) <================ This one! >>>> break; >>>> __set_current_state(TASK_UNINTERRUPTIBLE); >>>> mutex_unlock(&cpu_hotplug.lock); >>>> schedule(); >>>> } >>>> } >>> >>> I acutally just came to the same conclusion (7 hours of sleep later, the >>> mind indeed seems to be brighter ... what a poet I am). >>> >>> Lockdep doesn't know about this semantics of cpu_hotplug_begin(), and >>> therefore gets confused by the fact that mutual exclusion is actually >>> achieved through the refcount instead of mutex (and the same apparently >>> happened to me). >> >> No, that's not the problem. Lockdep is fine. The calltrace clearly shows that >> our refcounting has messed up somewhere. As a result, we really *are* running >> a hotplug-reader and a hotplug-writer at the same time! We really need to fix >> *that*! So please try the second debug patch I sent just now (with the BUG_ON() >> in put_online_cpus()). We need to know who is calling put_online_cpus() twice >> and fix that culprit! > > I don't think so. > > Lockdep is complaining, because > > (a) during system bootup, the smp_init() -> cpu_up() -> cpuup_callback() > teaches him about hotplug.lock -> slab_mutex dependency > > (b) many many jiffies later, nf_conntrack_cleanup_net() calls > kmem_cache_destroy(), which introduces slab_mutex -> hotplug.lock > dependency > > Lockdep rightfully (from his POV) sees this as potential ABBA, and reports > it, it's as simple as that. > It has no way of knowing the fact that the ABBA can actually never happen, > because of special semantics of cpu_hotplug.refcount and it's handling in > cpu_hotplug_begin(). >
Hmm, you are right.
> The "neither cpu_up() nor cpu_down() will proceed past cpu_hotplug_begin() > until everyone who called get_online_cpus() will call put_online_cpus()" > is totally invisible to lockdep.
I see your point..
> >>> So right, now I agree that the deadlock scenario I have come up with is >>> indeed bogus (*), and we just have to annotate this fact to lockdep >>> somehow. >> >> Yes, the deadlock scenario is bogus, but the refcounting leak is for real >> and needs fixing. > > With your patch applied, the BUG_ON() in put_online_cpus() didn't trigger > for me at all. Which is what I expected.
Oh, ok..
> >> I'm fine with this, but the real problem is elsewhere, like I mentioned above. >> This one is only a good-to-have, not a fix. >> >>> (*) I have seen machine locking hard reproducibly, but that was only with >>> additional Paul's patch, so I guess the lock order there actually was >>> wrong >> >> If refcounting was working fine, Paul's patch wouldn't have caused *any* issues. >> With that patch in place, the 2 places where rcu_barrier() get invoked (ie., >> kmem_cache_destroy() and deactivate_locked_super()) both start waiting on >> get_online_cpus() until the slab cpu hotplug notifier as well as the entire >> cpu_up operation completes. Absolutely no problem in that! So the fact that >> you are seeing lock-ups here is another indication that the problem is really >> elsewhere! > > I don't agree. The reason why Paul's patch (1331e7a1bb) started to trigger > this, is that (b) above doesn't exist in pre-1331e7a1bb kernels. >
So basically what you are saying is, the calltraces in the lockdep splat are from different points in time right? Then I see why its just a false positive and not a real bug. Regards, Srivatsa S. Bhat
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