[PATCH RFC tip/core/rcu 12/13] rcu: Document same-context read-side constraints

[Paul E. McKenney]

The intent is that a given RCU read-side critical section be confined
to a single context.  For example, it is illegal to invoke rcu_read_lock()
in an exception handler and then invoke rcu_read_unlock() from the
context of the task that received the exception.

Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
 include/linux/rcupdate.h |   15 +++++++++++++++
 include/linux/srcu.h     |    5 +++++
 2 files changed, 20 insertions(+), 0 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 5dd6fd8..81c04f4 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -265,6 +265,11 @@ extern int debug_lockdep_rcu_enabled(void);
  *
  * Checks debug_lockdep_rcu_enabled() to prevent false positives during boot
  * and while lockdep is disabled.
+ *
+ * Note that rcu_read_lock() and the matching rcu_read_unlock() must
+ * occur in the same context, for example, it is illegal to invoke
+ * rcu_read_unlock() in process context if the matching rcu_read_lock()
+ * was invoked from within an irq handler.
  */
 static inline int rcu_read_lock_held(void)
 {
@@ -689,6 +694,11 @@ static inline void rcu_read_unlock(void)
  * critical sections in interrupt context can use just rcu_read_lock(),
  * though this should at least be commented to avoid confusing people
  * reading the code.
+ *
+ * Note that rcu_read_lock_bh() and the matching rcu_read_unlock_bh()
+ * must occur in the same context, for example, it is illegal to invoke
+ * rcu_read_unlock_bh() from one task if the matching rcu_read_lock_bh()
+ * was invoked from some other task.
  */
 static inline void rcu_read_lock_bh(void)
 {
@@ -716,6 +726,11 @@ static inline void rcu_read_unlock_bh(void)
  * are being done using call_rcu_sched() or synchronize_rcu_sched().
  * Read-side critical sections can also be introduced by anything that
  * disables preemption, including local_irq_disable() and friends.
+ *
+ * Note that rcu_read_lock_sched() and the matching rcu_read_unlock_sched()
+ * must occur in the same context, for example, it is illegal to invoke
+ * rcu_read_unlock_sched() from process context if the matching
+ * rcu_read_lock_sched() was invoked from an NMI handler.
  */
 static inline void rcu_read_lock_sched(void)
 {
diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index d5334d0..83a4f06 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -158,6 +158,11 @@ static inline int srcu_read_lock_held(struct srcu_struct *sp)
  * one way to indirectly wait on an SRCU grace period is to acquire
  * a mutex that is held elsewhere while calling synchronize_srcu() or
  * synchronize_srcu_expedited().
+ *
+ * Note that srcu_read_lock() and the matching srcu_read_unlock() must
+ * occur in the same context, for example, it is illegal to invoke
+ * srcu_read_unlock() in an irq handler if the matching srcu_read_lock()
+ * was invoked in process context.
  */
 static inline int srcu_read_lock(struct srcu_struct *sp) __acquires(sp)
 {
-- 
1.7.3.2

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