Messages in this thread | | | Date | Wed, 20 May 2020 11:05:46 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [patch V6 12/37] x86/entry: Provide idtentry_entry/exit_cond_rcu() |
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On Wed, May 20, 2020 at 09:51:17AM -0700, Andy Lutomirski wrote: > 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?
From an RCU viewpoint, yes, give or take my concerns about someone putting rcu_tickle() on entry and rcu_irq_exit() on exit. Perhaps I can bring some lockdep trickery to bear.
But I must defer to Thomas and Peter on the non-RCU/non-NO_HZ_FULL portions of this.
Thanx, Paul
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