Messages in this thread | | | Date | Thu, 14 May 2015 13:08:23 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH RESEND 0/3] tracing/mm: Fix suspicious rcu_dereference_check() usage warnings |
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On Wed, 13 May 2015 13:17:06 +0530 Shreyas B Prabhu <shreyas@linux.vnet.ibm.com> wrote:
> >>> + TP_CONDITION(cpu_online(smp_processor_id())), > >> > >> Are we sure these can't generate check_preemption_disabled() warnings? > >> Is there some reason why all these calls always occur with preemption > >> disabled? > > > > Good catch. I don't think the code does. > > > > Now, I'm not sure if we should just add a raw_smp_processor_id(). The > > idea is just to make sure that the CPU we are running on is online, > > because it is possible to call theses trace points when the CPU is > > going offline. If that happens, then there's a race with RCU. > > > > Since a task can not be migrated to an offline CPU, we don't need to > > worry about the cpu_online(smp_processor_id()) returning a false > > positive. A false negative would just skip a tracepoint, but I'm not > > sure that is possible either. > > > > In any case, comments should also be added to why the condition is > > there. > > > I'll send a patch adding the comments.
Please also confirm that these tracepoints cannot be called from preemptible contexts. Or switch to raw_smp_processor_id().
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