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Date:   Thu, 13 Feb 2020 15:44:44 -0500
From:   Joel Fernandes <joel@joelfernandes.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        rostedt@goodmis.org, mingo@kernel.org, gregkh@linuxfoundation.org,
        gustavo@embeddedor.com, tglx@linutronix.de, josh@joshtriplett.org,
        mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com
Subject: Re: [PATCH v2 3/9] rcu,tracing: Create trace_rcu_{enter,exit}()
Message-ID: <20200213204444.GA94647@google.com>
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On Thu, Feb 13, 2020 at 10:56:12AM -0800, Paul E. McKenney wrote:
[...] 
> > > It might well be that I could make these functions be NMI-safe, but
> > > rcu_prepare_for_idle() in particular would be a bit ugly at best.
> > > So, before looking into that, I have a question.  Given these proposed
> > > changes, will rcu_nmi_exit_common() and rcu_nmi_enter_common() be able
> > > to just use in_nmi()?
> > 
> > That _should_ already be the case today. That is, if we end up in a
> > tracer and in_nmi() is unreliable we're already screwed anyway.
> 
> So something like this, then?  This is untested, probably doesn't even
> build, and could use some careful review from both Peter and Steve,
> at least.  As in the below is the second version of the patch, the first
> having been missing a couple of important "!" characters.

I removed the static from rcu_nmi_enter()/exit() as it is called from
outside, that makes it build now. Updated below is Paul's diff. I also added
NOKPROBE_SYMBOL() to rcu_nmi_exit() to match rcu_nmi_enter() since it seemed
asymmetric.

---8<-----------------------

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index d91c9156fab2e..bbcc7767f18ee 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -614,16 +614,18 @@ void rcu_user_enter(void)
 }
 #endif /* CONFIG_NO_HZ_FULL */
 
-/*
+/**
+ * rcu_nmi_exit - inform RCU of exit from NMI context
+ *
  * If we are returning from the outermost NMI handler that interrupted an
  * RCU-idle period, update rdp->dynticks and rdp->dynticks_nmi_nesting
  * to let the RCU grace-period handling know that the CPU is back to
  * being RCU-idle.
  *
- * If you add or remove a call to rcu_nmi_exit_common(), be sure to test
+ * If you add or remove a call to rcu_nmi_exit(), be sure to test
  * with CONFIG_RCU_EQS_DEBUG=y.
  */
-static __always_inline void rcu_nmi_exit_common(bool irq)
+__always_inline void rcu_nmi_exit(void)
 {
 	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 
@@ -651,25 +653,15 @@ static __always_inline void rcu_nmi_exit_common(bool irq)
 	trace_rcu_dyntick(TPS("Startirq"), rdp->dynticks_nmi_nesting, 0, atomic_read(&rdp->dynticks));
 	WRITE_ONCE(rdp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */
 
-	if (irq)
+	if (!in_nmi())
 		rcu_prepare_for_idle();
 
 	rcu_dynticks_eqs_enter();
 
-	if (irq)
+	if (!in_nmi())
 		rcu_dynticks_task_enter();
 }
-
-/**
- * rcu_nmi_exit - inform RCU of exit from NMI context
- *
- * If you add or remove a call to rcu_nmi_exit(), be sure to test
- * with CONFIG_RCU_EQS_DEBUG=y.
- */
-void rcu_nmi_exit(void)
-{
-	rcu_nmi_exit_common(false);
-}
+NOKPROBE_SYMBOL(rcu_nmi_exit);
 
 /**
  * rcu_irq_exit - inform RCU that current CPU is exiting irq towards idle
@@ -693,7 +685,7 @@ void rcu_nmi_exit(void)
 void rcu_irq_exit(void)
 {
 	lockdep_assert_irqs_disabled();
-	rcu_nmi_exit_common(true);
+	rcu_nmi_exit();
 }
 
 /*
@@ -777,7 +769,7 @@ void rcu_user_exit(void)
 #endif /* CONFIG_NO_HZ_FULL */
 
 /**
- * rcu_nmi_enter_common - inform RCU of entry to NMI context
+ * rcu_nmi_enter - inform RCU of entry to NMI context
  * @irq: Is this call from rcu_irq_enter?
  *
  * If the CPU was idle from RCU's viewpoint, update rdp->dynticks and
@@ -786,10 +778,10 @@ void rcu_user_exit(void)
  * long as the nesting level does not overflow an int.  (You will probably
  * run out of stack space first.)
  *
- * If you add or remove a call to rcu_nmi_enter_common(), be sure to test
+ * If you add or remove a call to rcu_nmi_enter(), be sure to test
  * with CONFIG_RCU_EQS_DEBUG=y.
  */
-static __always_inline void rcu_nmi_enter_common(bool irq)
+__always_inline void rcu_nmi_enter(void)
 {
 	long incby = 2;
 	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
@@ -807,12 +799,12 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
 	 */
 	if (rcu_dynticks_curr_cpu_in_eqs()) {
 
-		if (irq)
+		if (!in_nmi())
 			rcu_dynticks_task_exit();
 
 		rcu_dynticks_eqs_exit();
 
-		if (irq)
+		if (!in_nmi())
 			rcu_cleanup_after_idle();
 
 		incby = 1;
@@ -834,14 +826,6 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
 		   rdp->dynticks_nmi_nesting + incby);
 	barrier();
 }
-
-/**
- * rcu_nmi_enter - inform RCU of entry to NMI context
- */
-void rcu_nmi_enter(void)
-{
-	rcu_nmi_enter_common(false);
-}
 NOKPROBE_SYMBOL(rcu_nmi_enter);
 
 /**
@@ -869,7 +853,7 @@ NOKPROBE_SYMBOL(rcu_nmi_enter);
 void rcu_irq_enter(void)
 {
 	lockdep_assert_irqs_disabled();
-	rcu_nmi_enter_common(true);
+	rcu_nmi_enter();
 }
 
 /*