linux-kernel - Re: [PATCH v1 3/5] seqlock: seqcount_t: Implement all read APIs as statement expressions

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Message-ID: <20200828083022.GO1362448@hirez.programming.kicks-ass.net>
Date:   Fri, 28 Aug 2020 10:30:22 +0200
From:   peterz@...radead.org
To:     "Ahmed S. Darwish" <a.darwish@...utronix.de>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Sebastian A. Siewior" <bigeasy@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 3/5] seqlock: seqcount_t: Implement all read APIs as
 statement expressions

On Fri, Aug 28, 2020 at 03:07:08AM +0200, Ahmed S. Darwish wrote:
>  #define __read_seqcount_begin(s)					\
> +({									\
> +	unsigned seq;							\
> +									\
> +	do {								\
> +		seq = __seqcount_sequence(s);				\
> +		if (likely(! (seq & 1)))				\
> +			break;						\
> +		cpu_relax();						\
> +	} while (true);							\
> +									\
> +	kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);			\
> +	seq;								\
> +})

Since we're there anyway, does it make sense to (re)write this like:

	while ((seq = __seqcount_sequence(s)) & 1)
		cpu_relax();

?