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

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Message-ID: <20251005153008.GF2441659@ZenIV>
Date: Sun, 5 Oct 2025 16:30:08 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Oleg Nesterov <oleg@...hat.com>
Cc: 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>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/5] seqlock: introduce SEQLOCK_READ_SECTION()

On Sun, Oct 05, 2025 at 04:49:29PM +0200, Oleg Nesterov wrote:
> 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();

<cringe>

> 		}
> 
> 		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().

prepend_path() would be interesting to look at...