linux-kernel - [PATCH 0/5] seqlock: introduce SEQLOCK_READ

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Message-ID: <20251005144727.GA1188@redhat.com>
Date: Sun, 5 Oct 2025 16:47:27 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: 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>,
	Waiman Long <longman@...hat.com>, Will Deacon <will@...nel.org>
Cc: linux-kernel@...r.kernel.org
Subject: [PATCH 0/5] seqlock: introduce SEQLOCK_READ_SECTION()

OK, let me name it SEQLOCK_READ_SECTION().

Al, could you look at 5/5? Please nack it if you think it makes no sense
or wrong. I abused __dentry_path() to add another example. To simplify the
review, this is how it looks after the patch:

	static char *__dentry_path(const struct dentry *d, struct prepend_buffer *p)
	{
		const struct dentry *dentry;
		struct prepend_buffer b;

		rcu_read_lock();
		__SEQLOCK_READ_SECTION(&rename_lock, lockless, seq, NULL) {
			dentry = d;
			b = *p;
			while (!IS_ROOT(dentry)) {
				const struct dentry *parent = dentry->d_parent;

				prefetch(parent);
				if (!prepend_name(&b, &dentry->d_name))
					break;
				dentry = parent;
			}
			if (lockless)
				rcu_read_unlock();
		}

		if (b.len == p->len)
			prepend_char(&b, '/');
		return extract_string(&b);
	}


TODO: add another trivial helper

	static inline int need_seqretry_or_lock(seqlock_t *lock, int *seq)
	{
		int ret = !(*seq & 1) && read_seqretry(lock, *seq);

		if (ret)
			*seq = 1; /* make this counter odd */

		return ret;
	}

which can be used when the read section is more complex. Say, d_walk().

Oleg.
---

 fs/d_path.c             | 31 +++++++++++++------------------
 fs/proc/array.c         |  9 ++-------
 fs/proc/base.c          |  9 ++-------
 include/linux/seqlock.h | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
 kernel/sched/cputime.c  | 14 +++-----------
 5 files changed, 69 insertions(+), 43 deletions(-)