linux-kernel - Re: [RFC] locking: rwbase: Take care of ordering guarantee for fastpath reader

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Message-ID: <20210904101429.GB4323@worktop.programming.kicks-ass.net>
Date:   Sat, 4 Sep 2021 12:14:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Mike Galbraith <efault@....de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] locking: rwbase: Take care of ordering guarantee for
 fastpath reader

On Fri, Sep 03, 2021 at 10:50:58PM +0800, Boqun Feng wrote:
> > 	 * ordering, either is strong enough to provide ACQUIRE order
> > 	 * for the above load of @readers.
> > 	 */
> > 	rwbase_set_and_save_current_state(state);
> > 	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
> > 
> > 	while (readers) {
> > 		...
> > 		readers = atomic_read(&rwb->readers);
> 
> The above should be _acquire(), right? Pairs with the last reader
> exiting the critical section and dec ->readers to 0. If so, it
> undermines the necessity of the restructure?

This is the one that's followed by set_current_state(), no?