rcu: Document same-context read-side constraints

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@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 1eb520cd1680..e1b005918bbb 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)
 {