Messages in this thread | | | Date | Mon, 1 Sep 2014 13:20:59 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH RFC tip/core/rcu] Eliminate deadlock between CPU hotplug and expedited grace periods |
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On Thu, Aug 28, 2014 at 12:47:45PM -0700, Paul E. McKenney wrote: > Currently, the expedited grace-period primitives do get_online_cpus(). > This greatly simplifies their implementation, but means that calls to > them holding locks that are acquired by CPU-hotplug notifiers (to say > nothing of calls to these primitives from CPU-hotplug notifiers) can > deadlock. But this is starting to become inconvenient: > https://lkml.org/lkml/2014/8/5/754
Please recap the actual problem; the link might die and the actual mail linked to isn't very useful in any case.
> This commit avoids the deadlock and retains the simplicity by creating > a try_get_online_cpus(), which returns false if the get_online_cpus() > reference count could not immediately be incremented. If a call to > try_get_online_cpus() returns true, the expedited primitives operate > as before. If a call returns false, the expedited primitives fall back > to normal grace-period operations. This falling back of course results > in increased grace-period latency, but only during times when CPU > hotplug operations are actually in flight. The effect should therefore > be negligible during normal operation.
URGH.. I really hate that. The hotplug interface is already too horrible, we should not add such hacks to it.
How about ripping that rcu_expedited stuff out instead? That's all conditional anyhow, so might as well not do it.
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