On Mon, Sep 09, 2013 at 12:53:47PM +0200, Peter Zijlstra wrote: > 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.
Check this: 34240697d619c439c55f21989680024dcb604aab "rcu: Disable preemption in rcu_is_cpu_idle()" -- 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/
