[PATCH 2/6] rcu: Allow rcu_user_enter()/exit() to nest

[Frederic Weisbecker <fweisbec@gmail.com>]

Allow calls to rcu_user_enter() even if we are already
in userspace (as seen by RCU) and allow calls to rcu_user_exit()
even if we are already in the kernel.

This makes the APIs more flexible to be called from architectures.
Exception entries for example won't need to know if they come from
userspace before calling rcu_user_exit().

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Alessio Igor Bogani <abogani@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Avi Kivity <avi@redhat.com>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Geoff Levand <geoff@infradead.org>
Cc: Gilad Ben Yossef <gilad@benyossef.com>
Cc: Hakan Akkan <hakanakkan@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Kevin Hilman <khilman@ti.com>
Cc: Max Krasnyansky <maxk@qualcomm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Sven-Thorsten Dietrich <thebigcorporation@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
---
 kernel/rcutree.c |   38 ++++++++++++++++++++++++++++++--------
 1 files changed, 30 insertions(+), 8 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 64fc2cd..81a86ec 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -344,6 +344,10 @@ static int rcu_implicit_offline_qs(struct rcu_data *rdp)
 	return 0;
 }
 
+#ifdef CONFIG_RCU_USER_QS
+static DEFINE_PER_CPU(bool, in_user);
+#endif
+
 /*
  * rcu_eqs_enter_common - current CPU is moving towards extended quiescent state
  *
@@ -389,11 +393,9 @@ static void rcu_eqs_enter_common(struct rcu_dynticks *rdtp, long long oldval,
  */
 static void rcu_eqs_enter(bool user)
 {
-	unsigned long flags;
 	long long oldval;
 	struct rcu_dynticks *rdtp;
 
-	local_irq_save(flags);
 	rdtp = &__get_cpu_var(rcu_dynticks);
 	oldval = rdtp->dynticks_nesting;
 	WARN_ON_ONCE((oldval & DYNTICK_TASK_NEST_MASK) == 0);
@@ -402,7 +404,6 @@ static void rcu_eqs_enter(bool user)
 	else
 		rdtp->dynticks_nesting -= DYNTICK_TASK_NEST_VALUE;
 	rcu_eqs_enter_common(rdtp, oldval, user);
-	local_irq_restore(flags);
 }
 
 /**
@@ -419,7 +420,11 @@ static void rcu_eqs_enter(bool user)
  */
 void rcu_idle_enter(void)
 {
+	unsigned long flags;
+
+	local_irq_save(flags);
 	rcu_eqs_enter(0);
+	local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(rcu_idle_enter);
 
@@ -434,7 +439,16 @@ EXPORT_SYMBOL_GPL(rcu_idle_enter);
  */
 void rcu_user_enter(void)
 {
-	rcu_eqs_enter(1);
+	unsigned long flags;
+
+	WARN_ON_ONCE(!current->mm);
+
+	local_irq_save(flags);
+	if (!this_cpu_read(in_user)) {
+		this_cpu_write(in_user, true);
+		rcu_eqs_enter(1);
+	}
+	local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(rcu_user_enter);
 #endif
@@ -530,11 +544,9 @@ static void rcu_eqs_exit_common(struct rcu_dynticks *rdtp, long long oldval,
  */
 static void rcu_eqs_exit(bool user)
 {
-	unsigned long flags;
 	struct rcu_dynticks *rdtp;
 	long long oldval;
 
-	local_irq_save(flags);
 	rdtp = &__get_cpu_var(rcu_dynticks);
 	oldval = rdtp->dynticks_nesting;
 	WARN_ON_ONCE(oldval < 0);
@@ -543,7 +555,6 @@ static void rcu_eqs_exit(bool user)
 	else
 		rdtp->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
 	rcu_eqs_exit_common(rdtp, oldval, user);
-	local_irq_restore(flags);
 }
 
 /**
@@ -559,7 +570,11 @@ static void rcu_eqs_exit(bool user)
  */
 void rcu_idle_exit(void)
 {
+	unsigned long flags;
+
+	local_irq_save(flags);
 	rcu_eqs_exit(0);
+	local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(rcu_idle_exit);
 
@@ -572,7 +587,14 @@ EXPORT_SYMBOL_GPL(rcu_idle_exit);
  */
 void rcu_user_exit(void)
 {
-	rcu_eqs_exit(1);
+	unsigned long flags;
+
+	local_irq_save(flags);
+	if (this_cpu_read(in_user)) {
+		this_cpu_write(in_user, false);
+		rcu_eqs_exit(1);
+	}
+	local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(rcu_user_exit);
 #endif
-- 
1.7.5.4