[PATCH rcu 1/3] srcu: Guarantee non-negative return value from srcu_read_lock()

["Paul E. McKenney" <paulmck@kernel.org>]

For almost 20 years, the int return value from srcu_read_lock() has
been always either zero or one.  This commit therefore documents the
fact that it will be non-negative, and does the same for the underlying
__srcu_read_lock().

[ paulmck: Apply Andrii Nakryiko feedback. ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 include/linux/srcu.h  | 15 ++++++++-------
 kernel/rcu/srcutree.c |  3 ++-
 2 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 08339eb8a01c8..abaddd7e6ddf7 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -232,13 +232,14 @@ static inline int srcu_read_lock_held(const struct srcu_struct *ssp)
  * a mutex that is held elsewhere while calling synchronize_srcu() or
  * synchronize_srcu_expedited().
  *
- * 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.
+ * The return value from srcu_read_lock() is guaranteed to be
+ * non-negative.  This value 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)
 {
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 5e2e534647946..26ef58b481aae 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -738,7 +738,8 @@ EXPORT_SYMBOL_GPL(__srcu_check_read_flavor);
 /*
  * Counts the new reader in the appropriate per-CPU element of the
  * srcu_struct.
- * Returns an index that must be passed to the matching srcu_read_unlock().
+ * Returns a guaranteed non-negative index that must be passed to the
+ * matching __srcu_read_unlock().
  */
 int __srcu_read_lock(struct srcu_struct *ssp)
 {
-- 
2.40.1