On Fri, Sep 06, 2013 at 08:59:29PM +0200, Frederic Weisbecker wrote: > Imagine that you're running on an rcu read side critical section on CPU 0, > which > is not in extended quiescent state. Now you get preempted in the middle of > your > RCU read side critical section (you called rcu_read_lock() but not yet > rcu_read_unlock()). > > Later on, the task is woken up to be scheduled in CPU 1. If CPU 1 is in > extended > quiescent state because it runs is userspace, it receives a scheduler IPI, > then schedule_user() is called by the end of the interrupt and in turns calls > rcu_user_exit() > before the task is resumed to the code it was running on CPU 0, in the middle > of > the rcu read side extended quiescent state. > > See, the key here is the rcu_user_exit() that restore the CPU on RCU's state > machine. > There are other possible scheduler entrypoints when a CPU runs in user > extended quiescent > state: exception and syscall entries or even preempt_schedule_irq() in case > we receive an irq > in the kernel while we haven't yet reached the call to rcu_user_exit()... All > of these should > be covered, otherwise you bet RCU would be prompt to warn. > > That's why when we call rcu_is_cpu_idle() from an RCU read side critical > section, it's legit even > if we can be preempted anytime around it. > And preempt_disable() is probably not even necessary, except perhaps if > __get_cpu_var() itself > relies on non-preemptibility for its own correctness on the address > calculation.
I've tried reading that trice now, still not making much sense. In any case rcu_is_cpu_idle() is complete bollocks, either use __raw_get_cpu_var() and add a _coherent_ explanation for why its right, or its broken. In any case the preempt_disable/enable pair there is just plain wrong as Eric pointed out. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
