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Date:   Sat, 4 Sep 2021 12:12:19 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Boqun Feng <boqun.feng@gmail.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Will Deacon <will@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Mike Galbraith <efault@gmx.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [RFC] locking: rwbase: Take care of ordering guarantee for
 fastpath reader
Message-ID: <20210904101219.GA4323@worktop.programming.kicks-ass.net>
References: <20210901150627.620830-1-boqun.feng@gmail.com>
 <YTC7sariSyBW48nh@hirez.programming.kicks-ass.net>
 <YTI2UjKy+C7LeIf+@boqun-archlinux>
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On Fri, Sep 03, 2021 at 10:50:58PM +0800, Boqun Feng wrote:
> On Thu, Sep 02, 2021 at 01:55:29PM +0200, Peter Zijlstra wrote:
> [...]
> > >  	raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
> > >  	rwbase_rtmutex_unlock(rtm);
> > >  }
> > > @@ -216,8 +229,14 @@ static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
> > >  	 */
> > >  	rwbase_set_and_save_current_state(state);
> > >  
> > > -	/* Block until all readers have left the critical section. */
> > > -	for (; atomic_read(&rwb->readers);) {
> > > +	/*
> > > +	 * Block until all readers have left the critical section.
> > > +	 *
> > > +	 * _acqurie() is needed in case that the reader side runs in the fast
> > > +	 * path, pairing with the atomic_dec_and_test() in rwbase_read_unlock(),
> > > +	 * provides ACQUIRE.
> > > +	 */
> > > +	for (; atomic_read_acquire(&rwb->readers);) {
> > >  		/* Optimized out for rwlocks */
> > >  		if (rwbase_signal_pending_state(state, current)) {
> > >  			__set_current_state(TASK_RUNNING);
> > 
> > I think we can restructure things to avoid this one, but yes. Suppose we
> > do:
> > 
> > 	readers = atomic_sub_return_relaxed(READER_BIAS, &rwb->readers);
> > 
> > 	/*
> > 	 * These two provide either an smp_mb() or an UNLOCK+LOCK
> 
> By "UNLOCK+LOCK", you mean unlock(->pi_lock) + lock(->wait_lock), right?
> This may be unrelated, but in our memory model only unlock+lock pairs on
> the same lock provide TSO-like ordering ;-) IOW, unlock(->pi_lock) +
> lock(->wait_lock) on the same CPU doesn't order reads before and after.

Hurpm.. what actual hardware does that? PPC uses LWSYNC for
ACQUIRE/RELEASE, and ARM64 has RCsc RELEASE+ACQUIRE ordering.

Both should provide RC-TSO (or stronger) for UNLOCK-A + LOCK-B.