Messages in this thread | | | Date | Mon, 9 Sep 2013 14:13:31 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical section? |
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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()"
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