diff options
| author | Oleg Nesterov <oleg@redhat.com> | 2025-09-28 18:20:29 +0200 |
|---|---|---|
| committer | Peter Zijlstra <peterz@infradead.org> | 2025-10-21 12:31:56 +0200 |
| commit | 28a0ee311960baad97bf85e1e995aed4a71e22a2 (patch) | |
| tree | fadecc4815fde6027e94fc75f89619ad6cf597ed | |
| parent | 44472d1b83127e579c798ff92a07ae86d98b61b9 (diff) | |
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@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
| -rw-r--r-- | Documentation/locking/seqlock.rst | 9 |
1 files changed, 5 insertions, 4 deletions
diff --git a/Documentation/locking/seqlock.rst b/Documentation/locking/seqlock.rst index 3fb7ea3ab22a..9899871d3d9a 100644 --- a/Documentation/locking/seqlock.rst +++ b/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]] ... */ |