Messages in this thread | | | Date | Fri, 7 Dec 2012 20:56:29 +0100 | From | Oleg Nesterov <> | Subject | Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline |
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On 12/07, Srivatsa S. Bhat wrote: > > On 12/06/2012 09:48 PM, Oleg Nesterov wrote: > > On 12/06, Srivatsa S. Bhat wrote: > >> > >> +void get_online_cpus_atomic(void) > >> +{ > >> + int c, old; > >> + > >> + preempt_disable(); > >> + read_lock(&hotplug_rwlock); > > > > Confused... Why it also takes hotplug_rwlock? > > To avoid ABBA deadlocks. > > hotplug_rwlock was meant for the "light" readers. > The atomic counters were meant for the "heavy/full" readers.
OK, I got lost a bit. I'll try to read v3 tomorrow.
> > Obviously you can't use get_online_cpus_atomic() under rq->lock or > > task->pi_lock or any other lock CPU_DYING can take. Probably this is > > fine, but perhaps it makes sense to add the lockdep annotations. > > Hmm, you are right. We can't use _atomic() in the CPU_DYING path.
Not sure I undestand... I simply meant that, say, get_online_cpus_atomic() under task->pi_lock can obviously deadlock with take_cpu_down() which can want the same task->pi_lock after disable_atomic_reader().
Oleg.
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