rcu: Is it safe to enter an RCU read-side critical section?
There is currently no way for kernel code to determine whether it
is safe to enter an RCU read-side critical section, in other words,
whether or not RCU is paying attention to the currently running CPU.
Given the large and increasing quantity of code shared by the idle loop
and non-idle code, the this shortcoming is becoming increasingly painful.
This commit therefore adds rcu_watching_this_cpu(), which returns true
if it is safe to enter an RCU read-side critical section on the currently
running CPU. This function is quite fast, using only a __this_cpu_read().
However, the caller must disable preemption.
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 5b444e0..a41eb35 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -261,6 +261,10 @@
rcu_irq_exit(); \
} while (0)
+#if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) || defined(CONFIG_SMP)
+extern int rcu_is_cpu_idle(void);
+#endif /* #if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) || defined(CONFIG_SMP) */
+
/*
* Infrastructure to implement the synchronize_() primitives in
* TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
@@ -297,10 +301,6 @@
}
#endif /* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
-#if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_SMP)
-extern int rcu_is_cpu_idle(void);
-#endif /* #if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_SMP) */
-
#if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU)
bool rcu_lockdep_current_cpu_online(void);
#else /* #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index e31005e..67fe672 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -132,4 +132,13 @@
}
#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
+#ifdef CONFIG_RCU_TRACE
+
+static inline bool rcu_watching_this_cpu(void)
+{
+ return !rcu_is_cpu_idle();
+}
+
+#endif /* #ifdef CONFIG_RCU_TRACE */
+
#endif /* __LINUX_RCUTINY_H */
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 226169d..c605b41 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -90,4 +90,6 @@
extern void rcu_scheduler_starting(void);
extern int rcu_scheduler_active __read_mostly;
+extern bool rcu_watching_this_cpu(void);
+
#endif /* __LINUX_RCUTREE_H */
diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
index 7e3b0d6..b14701f 100644
--- a/kernel/rcutiny.c
+++ b/kernel/rcutiny.c
@@ -176,7 +176,7 @@
}
EXPORT_SYMBOL_GPL(rcu_irq_enter);
-#ifdef CONFIG_DEBUG_LOCK_ALLOC
+#if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE)
/*
* Test whether RCU thinks that the current CPU is idle.
@@ -187,7 +187,7 @@
}
EXPORT_SYMBOL(rcu_is_cpu_idle);
-#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
+#endif /* defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) */
/*
* Test whether the current CPU was interrupted from idle. Nested
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index a06d172..38c6883 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -666,6 +666,19 @@
}
EXPORT_SYMBOL(rcu_is_cpu_idle);
+/**
+ * rcu_watching_this_cpu - are RCU read-side critical sections safe?
+ *
+ * Return true if RCU is watching the running CPU, which means that
+ * this CPU can safely enter RCU read-side critical sections. Unlike
+ * rcu_is_cpu_idle(), the caller of rcu_watching_this_cpu() must have at
+ * least disabled preemption.
+ */
+bool rcu_watching_this_cpu(void)
+{
+ return !!__this_cpu_read(rcu_dynticks.dynticks_nesting);
+}
+
#if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU)
/*
