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: <20251002125823.GC32506@redhat.com>
Date: Thu, 2 Oct 2025 14:58:24 +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)

Or, better

	static inline int xxx(seqlock_t *lock, int *seq, unsigned long *flags)
	{
		int retry = 0;

		if (*seq & 1) {
			if (flags)
				read_sequnlock_excl_irqrestore(lock, *flags);
			else
				read_sequnlock_excl(lock);
		} else if (read_seqretry(lock, *seq)) {
			retry = *seq = 1;
			if (flags)
				read_seqlock_excl_irqsave(lock, *flags);
			else
				read_seqlock_excl(lock);
		}

		return retry;
	}

	#define __XXX(lock, lockless, seq, flags)			\
		for (int lockless = 1, seq = read_seqbegin(lock);	\
		     lockless || xxx(lock, &seq, flags);		\
		     lockless = 0)

	#define XXX(lock, flags)	\
		__XXX(lock, __UNIQUE_ID(lockless), __UNIQUE_ID(seq), flags)


With this version the change in thread_group_cputime() even shrinks .text a little bit.

I'd like to send the patch, but I still can't find a good name... Peter, will you agree
with SEQBEGIN_OR_LOCK(lock, flags) ?

Oleg.