Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical section?

[Peter Zijlstra <peterz@infradead.org>]

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.