linux-kernel - [PATCH rcu] srcu: Guarantee non-negative return value from srcu_read

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Message-ID: <97594073-e296-4876-9d6a-1e4a4f33d857@paulmck-laptop>
Date: Mon, 21 Oct 2024 15:13:05 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: rcu@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, kernel-team@...a.com, rostedt@...dmis.org,
	peterz@...radead.org, andrii@...nel.org
Subject: [PATCH rcu] srcu: Guarantee non-negative return value from
 srcu_read_lock()

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.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Andrii Nakryiko <andrii@...nel.org

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index bab1dae3f69e6..512a8c54ba5ba 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -238,13 +238,14 @@ void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor);
  * 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)
 {