linux-kernel - Re: [PATCH V2] MIPS: implement smp_cond_load

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Date:   Wed, 11 Jul 2018 13:09:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>
Cc:     Jiaxun Yang <jiaxun.yang@...goat.com>, linux-mips@...ux-mips.org,
        陈华才 <chenhc@...ote.com>,
        Paul Burton <paul.burton@...s.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>,
        Fuxin Zhang <zhangfx@...ote.com>,
        wuzhangjin <wuzhangjin@...il.com>,
        stable <stable@...r.kernel.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Akira Yokosawa <akiyks@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2] MIPS: implement smp_cond_load_acquire() for Loongson-3

On Wed, Jul 11, 2018 at 11:21:06AM +0100, Will Deacon wrote:
> On Wed, Jul 11, 2018 at 06:05:51PM +0800, Jiaxun Yang wrote:
> > On 2018-7-10 Tue at 20:17:27，Peter Zijlstra Wrote：

> > > Still, even with the rules above, the best work-around is still the very
> > > same cpu_relax() hack.
> > 
> > As you say, SFB makes Loongson not fully SMP-coherent.
> > However, modify cpu_relax can solve the current problem,
> > but not so straight forward. On the other hand, providing a Loongson-specific 
> > WRITE_ONCE looks more reasonable, because it the eliminate the "non-cohrency".
> > So we can solve the bug from the root.
> 
> Curious, but why is it not straight-forward to hack cpu_relax()? If you try
> to hack WRITE_ONCE, you also need to hack atomic_set, atomic64_set and all
> the places that should be using WRITE_ONCE but aren't ;~)

Right.

The problem isn't stores pre-se, normal progress should contain enough
stores to flush out 'old' bits in the natural order of things. But the
problem is spin-wait loops that inhibit normal progress (and thereby
store-buffer flushing).

And all spin-wait loops should be having cpu_relax() in them. So
cpu_relax() is the natural place to fix this.

Adding SYNC to WRITE_ONCE()/atomic* will hurt performance lots and will
ultimately not guarantee anything more; and as Will said, keep you
chasing dragons where people forgot to use WRITE_ONCE() where they maybe
should've.