On Wed, May 20, 2020 at 8:36 AM Andy Lutomirski <l...@kernel.org> wrote:
>
> On Tue, May 19, 2020 at 7:23 PM Paul E. McKenney <paul...@kernel.org> wrote:
> >
> > On Tue, May 19, 2020 at 05:26:58PM -0700, Andy Lutomirski wrote:
> > > On Tue, May 19, 2020 at 2:20 PM Thomas Gleixner <t...@linutronix.de>
> > > wrote:
First, the patch as you submitted it is Acked-by: Andy Lutomirski
<l...@kernel.org>. I think there are cleanups that should happen, but
I think the patch is correct.
About cleanups, concretely: I think that everything that calls
__idtenter_entry() is called in one of a small number of relatively
sane states:
1. User mode. This is easy.
2. Kernel, RCU is watching, everything is sane. We don't actually
need to do any RCU entry/exit pairs -- we should be okay with just a
hypothetical RCU tickle (and IRQ tracing, etc). This variant can
sleep after the entry part finishes if regs->flags & IF and no one
turned off preemption.
3. Kernel, RCU is not watching, system was idle. This can only be an
actual interrupt.
So maybe the code can change to:
if (user_mode(regs)) {
enter_from_user_mode();
} else {
if (!__rcu_is_watching()) {
/*
* If RCU is not watching then the same careful
* sequence vs. lockdep and tracing is required.
*
* This only happens for IRQs that hit the idle loop, and
* even that only happens if we aren't using the sane
* MWAIT-while-IF=0 mode.
*/
lockdep_hardirqs_off(CALLER_ADDR0);
rcu_irq_enter();
instrumentation_begin();
trace_hardirqs_off_prepare();
instrumentation_end();
return true;
} else {
/*
* If RCU is watching then the combo function
* can be used.
*/
instrumentation_begin();
trace_hardirqs_off();
rcu_tickle();
instrumentation_end();
}
}
return false;
This is exactly what you have except that the cond_rcu part is gone
and I added rcu_tickle().
Paul, the major change here is that if an IRQ hits normal kernel code
(i.e. code where RCU is watching and we're not in an EQS), the IRQ
won't call rcu_irq_enter() and rcu_irq_exit(). Instead it will call
rcu_tickle() on entry and nothing on exit. Does that cover all the
bases?
