[PATCH 1/2] seqlock: fix the wrong read_seqbegin_or_lock/need_seqretry documentation

[Oleg Nesterov <oleg@redhat.com>]

Half of the read_seqbegin_or_lock's users are buggy (I'll send the
fixes), and I guess this is because the documentation and the pseudo
code in Documentation/locking/seqlock.rst are wrong.

Pseudo code:

	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() returns with the even seq, 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@redhat.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 bfda1a5fecad..4bdf8d4ed2a2 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