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Message-ID: <b83e9c1d-2623-4abf-8c63-1110a0b92d2e@redhat.com>
Date: Wed, 1 Oct 2025 14:21:11 -0400
From: Waiman Long <llong@...hat.com>
To: Oleg Nesterov <oleg@...hat.com>, 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>, Will Deacon <will@...nel.org>
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] documentation: seqlock: fix the wrong documentation
of read_seqbegin_or_lock/need_seqretry
On 9/28/25 12:20 PM, Oleg Nesterov wrote:
> 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]] ... */
It is kind of odd to initialize the sequence to 1 and add an sequence
increment inside the loop. Perhaps we can do something like:
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 5ce48eab7a2a..0f607ef28d98 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -1126,6 +1126,9 @@ read_sequnlock_excl_irqrestore(seqlock_t *sl,
unsigned long flags)
*/
static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
{
+ if (!(*seq & 1)) /* Reread sequence # if even */
+ *seq = seqprop_sequence(&lock->seqcount);
+
if (!(*seq & 1)) /* Even */
*seq = read_seqbegin(lock);
else /* Odd */
@@ -1144,6 +1147,15 @@ static inline int need_seqretry(seqlock_t *lock,
int seq)
return !(seq & 1) && read_seqretry(lock, seq);
}
+static inline int need_seqretry_once(seqlock_t *lock, int *seq)
+{
+ int ret = !(*seq & 1) && read_seqretry(lock, *seq);
+
+ if (ret)
+ *seq = 1; /* Enforce locking in next iteration */
+ return ret;
+}
+
With this, the current document should be good. Users have the option of
using need_seqretry_once() to enforce at most 1 iteration. Of course, we
still need to do similar change to the other read_seqbegin_or_lock_*()
variants.
My 2 cents.
Cheers,
Longman
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