linux-kernel - Re: [RFC 2/1] seqlock: make the read_seqbegin_or_lock() API more simple and less error-prone ?

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Message-ID: <20251001134628.GD20441@redhat.com>
Date: Wed, 1 Oct 2025 15:46:31 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
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>,
	Waiman Long <longman@...hat.com>, Will Deacon <will@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC 2/1] seqlock: make the read_seqbegin_or_lock() API more
 simple and less error-prone ?

On 10/01, Oleg Nesterov wrote:
>
> +static inline int xxx(seqlock_t *lock, int lockless, int *seq, unsigned long *flags)
> +{
> +	if (lockless) {
> +		*seq = read_seqbegin(lock);
> +		return 1;
> +	} else if (*seq & 1) {
> +		if (flags)
> +			read_sequnlock_excl_irqrestore(lock, *flags);
> +		else
> +			read_sequnlock_excl(lock);
> +		return 0;
> +	} else if (read_seqretry(lock, *seq)) {
> +		if (flags)
> +			read_seqlock_excl_irqsave(lock, *flags);
> +		else
> +			read_seqlock_excl(lock);
> +		*seq = 1;
> +		return 1;
> +	} else {
> +		return 0;
> +	}
> +}
> +
> +#define __XXX(lock, lockless, seq, flags)	\
> +	for (int lockless = 1, seq; xxx(lock, lockless, &seq, flags); lockless = 0)
> +
> +#define XXX(lock, flags)	\
> +	__XXX(lock, __UNIQUE_ID(lockless), __UNIQUE_ID(seq), flags)

Note that __XXX() can have users too. See the patch below. Not that it makes a
lot of sense, just for example.

Oleg.


diff --git a/fs/d_path.c b/fs/d_path.c
index bb365511066b..b4cde188be4f 100644
--- a/fs/d_path.c
+++ b/fs/d_path.c
@@ -332,28 +332,23 @@ static char *__dentry_path(const struct dentry *d, struct prepend_buffer *p)
 {
 	const struct dentry *dentry;
 	struct prepend_buffer b;
-	int seq = 0;
 
 	rcu_read_lock();
-restart:
-	dentry = d;
-	b = *p;
-	read_seqbegin_or_lock(&rename_lock, &seq);
-	while (!IS_ROOT(dentry)) {
-		const struct dentry *parent = dentry->d_parent;
+	__XXX(&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 (!(seq & 1))
-		rcu_read_unlock();
-	if (need_seqretry(&rename_lock, seq)) {
-		seq = 1;
-		goto restart;
+			prefetch(parent);
+			if (!prepend_name(&b, &dentry->d_name))
+				break;
+			dentry = parent;
+		}
+		if (lockless)
+			rcu_read_unlock();
 	}
-	done_seqretry(&rename_lock, seq);
+
 	if (b.len == p->len)
 		prepend_char(&b, '/');
 	return extract_string(&b);