This commit adds some additional usage constraints to the kernel-doc
headers of srcu_read_lock() and srcu_read_lock_nmi_safe().

Suggested-by: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 include/linux/srcu.h | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 06728ef6f32a4..46fd06b212baa 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -235,10 +235,13 @@ static inline void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flav
  * 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.
+ * The return value from srcu_read_lock() must be passed unaltered
+ * to the matching srcu_read_unlock().  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.  Or,
+ * for that matter to invoke srcu_read_unlock() from one task and the
+ * matching srcu_read_lock() from another.
  */
 static inline int srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp)
 {
@@ -256,6 +259,10 @@ static inline int srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp)
  *
  * Enter an SRCU read-side critical section, but in an NMI-safe manner.
  * See srcu_read_lock() for more information.
+ *
+ * If srcu_read_lock_nmisafe() is ever used on an srcu_struct structure,
+ * then none of the other flavors may be used, whether before, during,
+ * or after.
  */
 static inline int srcu_read_lock_nmisafe(struct srcu_struct *ssp) __acquires(ssp)
 {
-- 
2.40.1