Messages in this thread | | | From | Andy Lutomirski <> | Date | Wed, 20 May 2020 09:51:17 -0700 | Subject | Re: [patch V6 12/37] x86/entry: Provide idtentry_entry/exit_cond_rcu() |
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On Wed, May 20, 2020 at 8:36 AM Andy Lutomirski <luto@kernel.org> wrote: > > On Tue, May 19, 2020 at 7:23 PM Paul E. McKenney <paulmck@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 <tglx@linutronix.de> wrote:
First, the patch as you submitted it is Acked-by: Andy Lutomirski <luto@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?
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