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@xxxxxxxxxx>
---
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
