linux-kernel - [PATCH tip/core/rcu 07/17] rcu: Eliminate rcu_irq_enter

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Date:   Fri,  1 Dec 2017 11:36:38 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 07/17] rcu: Eliminate rcu_irq_enter_disabled()

Now that the irq path uses the rcu_nmi_{enter,exit}() algorithm,
rcu_irq_enter() and rcu_irq_exit() may be used from any context.  There is
thus no need for rcu_irq_enter_disabled() and for the checks using it.
This commit therefore eliminates rcu_irq_enter_disabled().

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 include/linux/rcutiny.h    |  1 -
 include/linux/rcutree.h    |  1 -
 include/linux/tracepoint.h |  5 +----
 kernel/rcu/tree.c          | 22 ++--------------------
 kernel/trace/trace.c       | 11 -----------
 5 files changed, 3 insertions(+), 37 deletions(-)

diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index b3dbf9502fd0..ce9beec35e34 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -111,7 +111,6 @@ static inline void rcu_cpu_stall_reset(void) { }
 static inline void rcu_idle_enter(void) { }
 static inline void rcu_idle_exit(void) { }
 static inline void rcu_irq_enter(void) { }
-static inline bool rcu_irq_enter_disabled(void) { return false; }
 static inline void rcu_irq_exit_irqson(void) { }
 static inline void rcu_irq_enter_irqson(void) { }
 static inline void rcu_irq_exit(void) { }
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 37d6fd3b7ff8..fd996cdf1833 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -85,7 +85,6 @@ void rcu_irq_enter(void);
 void rcu_irq_exit(void);
 void rcu_irq_enter_irqson(void);
 void rcu_irq_exit_irqson(void);
-bool rcu_irq_enter_disabled(void);
 
 void exit_rcu(void);
 
diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index a26ffbe09e71..c94f466d57ef 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -137,11 +137,8 @@ extern void syscall_unregfunc(void);
 									\
 		if (!(cond))						\
 			return;						\
-		if (rcucheck) {						\
-			if (WARN_ON_ONCE(rcu_irq_enter_disabled()))	\
-				return;					\
+		if (rcucheck)						\
 			rcu_irq_enter_irqson();				\
-		}							\
 		rcu_read_lock_sched_notrace();				\
 		it_func_ptr = rcu_dereference_sched((tp)->funcs);	\
 		if (it_func_ptr) {					\
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index d123474fe829..444aa2b3f24d 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -271,20 +271,6 @@ static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
 };
 
 /*
- * There's a few places, currently just in the tracing infrastructure,
- * that uses rcu_irq_enter() to make sure RCU is watching. But there's
- * a small location where that will not even work. In those cases
- * rcu_irq_enter_disabled() needs to be checked to make sure rcu_irq_enter()
- * can be called.
- */
-static DEFINE_PER_CPU(bool, disable_rcu_irq_enter);
-
-bool rcu_irq_enter_disabled(void)
-{
-	return this_cpu_read(disable_rcu_irq_enter);
-}
-
-/*
  * Record entry into an extended quiescent state.  This is only to be
  * called when not already in an extended quiescent state.
  */
@@ -792,10 +778,8 @@ static void rcu_eqs_enter_common(bool user)
 		do_nocb_deferred_wakeup(rdp);
 	}
 	rcu_prepare_for_idle();
-	__this_cpu_inc(disable_rcu_irq_enter);
-	rdtp->dynticks_nesting = 0; /* Breaks tracing momentarily. */
-	rcu_dynticks_eqs_enter(); /* After this, tracing works again. */
-	__this_cpu_dec(disable_rcu_irq_enter);
+	rdtp->dynticks_nesting = 0;
+	rcu_dynticks_eqs_enter();
 	rcu_dynticks_task_enter();
 
 	/*
@@ -1001,10 +985,8 @@ static void rcu_eqs_exit(bool user)
 	if (oldval) {
 		rdtp->dynticks_nesting++;
 	} else {
-		__this_cpu_inc(disable_rcu_irq_enter);
 		rcu_eqs_exit_common(1, user);
 		rdtp->dynticks_nesting = 1;
-		__this_cpu_dec(disable_rcu_irq_enter);
 		WRITE_ONCE(rdtp->dynticks_nmi_nesting, DYNTICK_IRQ_NONIDLE);
 	}
 }
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 73e67b68c53b..dbce1be3bab8 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2682,17 +2682,6 @@ void __trace_stack(struct trace_array *tr, unsigned long flags, int skip,
 	if (unlikely(in_nmi()))
 		return;
 
-	/*
-	 * It is possible that a function is being traced in a
-	 * location that RCU is not watching. A call to
-	 * rcu_irq_enter() will make sure that it is, but there's
-	 * a few internal rcu functions that could be traced
-	 * where that wont work either. In those cases, we just
-	 * do nothing.
-	 */
-	if (unlikely(rcu_irq_enter_disabled()))
-		return;
-
 	rcu_irq_enter_irqson();
 	__ftrace_trace_stack(buffer, flags, skip, pc, NULL);
 	rcu_irq_exit_irqson();
-- 
2.5.2