linux-kernel - [PATCH 1/1] documentation: seqlock: fix the wrong documentation of read_seqbegin_or_lock/need

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Message-ID: <20250928162029.GA3121@redhat.com>
Date: Sun, 28 Sep 2025 18:20:29 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Boqun Feng <boqun.feng@...il.com>, David Howells <dhowells@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, Li RongQing <lirongqing@...du.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Waiman Long <longman@...hat.com>, Will Deacon <will@...nel.org>
Cc: linux-kernel@...r.kernel.org
Subject: [PATCH 1/1] documentation: seqlock: fix the wrong documentation of
 read_seqbegin_or_lock/need_seqretry

The comments and pseudo code in Documentation/locking/seqlock.rst are wrong:

	int seq = 0;
	do {
		read_seqbegin_or_lock(&foo_seqlock, &seq);

		/* ... [[read-side critical section]] ... */

	} while (need_seqretry(&foo_seqlock, seq));

read_seqbegin_or_lock() always returns with an even "seq" and need_seqretry()
doesn't change this counter. This means that seq is always even and thus the
locking pass is simply impossible.

IOW, "_or_lock" has no effect and this code doesn't differ from

	do {
		seq = read_seqbegin(&foo_seqlock);

		/* ... [[read-side critical section]] ... */

	} while (read_seqretry(&foo_seqlock, seq));

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 Documentation/locking/seqlock.rst | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/Documentation/locking/seqlock.rst b/Documentation/locking/seqlock.rst
index ec6411d02ac8..167d442d3c7f 100644
--- a/Documentation/locking/seqlock.rst
+++ b/Documentation/locking/seqlock.rst
@@ -218,13 +218,14 @@ Read path, three categories:
    according to a passed marker. This is used to avoid lockless readers
    starvation (too much retry loops) in case of a sharp spike in write
    activity. First, a lockless read is tried (even marker passed). If
-   that trial fails (odd sequence counter is returned, which is used as
-   the next iteration marker), the lockless read is transformed to a
-   full locking read and no retry loop is necessary::
+   that trial fails (sequence counter doesn't match), make the marker
+   odd for the next iteration, the lockless read is transformed to a
+   full locking read and no retry loop is necessary, for example::
 
 	/* marker; even initialization */
-	int seq = 0;
+	int seq = 1;
 	do {
+		seq++; /* 2 on the 1st/lockless path, otherwise odd */
 		read_seqbegin_or_lock(&foo_seqlock, &seq);
 
 		/* ... [[read-side critical section]] ... */
-- 
2.25.1.362.g51ebf55