linux-kernel - [tip: locking/core] jump_label: Clarify condition in static_key_fast_inc_not

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Date: Mon, 17 Jun 2024 15:47:18 -0000
From: "tip-bot2 for Thomas Gleixner" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Thomas Gleixner <tglx@...utronix.de>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: locking/core] jump_label: Clarify condition in
 static_key_fast_inc_not_disabled()

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     695ef796467ed228b60f1915995e390aea3d85c6
Gitweb:        https://git.kernel.org/tip/695ef796467ed228b60f1915995e390aea3d85c6
Author:        Thomas Gleixner <tglx@...utronix.de>
AuthorDate:    Mon, 10 Jun 2024 14:46:37 +02:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Tue, 11 Jun 2024 11:25:23 +02:00

jump_label: Clarify condition in static_key_fast_inc_not_disabled()

The second part of

      if (v <= 0 || (v + 1) < 0)

is not immediately obvious that it acts as overflow protection.

Check explicitely for v == INT_MAX instead and add a proper comment how
this is used at the call sites.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20240610124406.484973160@linutronix.de
---
 kernel/jump_label.c |  9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index 1f05a19..4d06ec2 100644
--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -132,12 +132,15 @@ bool static_key_fast_inc_not_disabled(struct static_key *key)
 	/*
 	 * Negative key->enabled has a special meaning: it sends
 	 * static_key_slow_inc/dec() down the slow path, and it is non-zero
-	 * so it counts as "enabled" in jump_label_update().  Note that
-	 * atomic_inc_unless_negative() checks >= 0, so roll our own.
+	 * so it counts as "enabled" in jump_label_update().
+	 *
+	 * The INT_MAX overflow condition is either used by the networking
+	 * code to reset or detected in the slow path of
+	 * static_key_slow_inc_cpuslocked().
 	 */
 	v = atomic_read(&key->enabled);
 	do {
-		if (v <= 0 || (v + 1) < 0)
+		if (v <= 0 || v == INT_MAX)
 			return false;
 	} while (!likely(atomic_try_cmpxchg(&key->enabled, &v, v + 1)));