[patch V4 part 3 11/29] rcu: Provide rcu_irq_exit_preempt()

Thomas Gleixner
2020-05-05T14:21:12Z

Interrupts and exceptions invoke rcu_irq_enter() on entry and need to
invoke rcu_irq_exit() before they either return to the interrupted code or
invoke the scheduler due to preemption.

The general assumption is that RCU idle code has to have preemption
disabled so that a return from interrupt cannot schedule. So the return
from interrupt code invokes rcu_irq_exit() and preempt_schedule_irq().

If there is any imbalance in the rcu_irq/nmi* invocations or RCU idle code
had preemption enabled then this goes unnoticed until the CPU goes idle or
some other RCU check is executed.

Provide rcu_irq_exit_preempt() which can be invoked from the
interrupt/exception return code in case that preemption is enabled. It
invokes rcu_irq_exit() and contains a few sanity checks in case that
CONFIG_PROVE_RCU is enabled to catch such issues directly.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
---
include/linux/rcutiny.h | 1 +
include/linux/rcutree.h | 1 +
kernel/rcu/tree.c | 21 +++++++++++++++++++++
3 files changed, 23 insertions(+)

--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -71,6 +71,7 @@ static inline void rcu_irq_enter(void) {
static inline void rcu_irq_exit_irqson(void) { }
static inline void rcu_irq_enter_irqson(void) { }
static inline void rcu_irq_exit(void) { }
+static inline void rcu_irq_exit_preempt(void) { }
static inline void exit_rcu(void) { }
static inline bool rcu_preempt_need_deferred_qs(struct task_struct *t)
{
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -46,6 +46,7 @@ void rcu_idle_enter(void);
void rcu_idle_exit(void);
void rcu_irq_enter(void);
void rcu_irq_exit(void);
+void rcu_irq_exit_preempt(void);
void rcu_irq_enter_irqson(void);
void rcu_irq_exit_irqson(void);

--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -706,6 +706,27 @@ void noinstr rcu_irq_exit(void)
rcu_nmi_exit();
}

+/**
+ * rcu_irq_exit_preempt - Inform RCU that current CPU is exiting irq
+ * towards in kernel preemption
+ *
+ * Same as rcu_irq_exit() but has a sanity check that scheduling is safe
+ * from RCU point of view. Invoked from return from interrupt before kernel
+ * preemption.
+ */
+void rcu_irq_exit_preempt(void)
+{
+ lockdep_assert_irqs_disabled();
+ rcu_nmi_exit();
+
+ RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nesting) <= 0,
+ "RCU dynticks_nesting counter underflow/zero!");
+ RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nmi_nesting) <= 0,
+ "RCU dynticks_nmi_nesting counter underflow/zero!");
+ RCU_LOCKDEP_WARN(rcu_dynticks_curr_cpu_in_eqs(),
+ "RCU in extended quiescent state!");
+}
+
/*
* Wrapper for rcu_irq_exit() where interrupts are enabled.
*