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 <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>

Documentation/locking/seqlock.rst

@@ -220,13 +220,14 @@ 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]] ... */