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

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Date:   Wed, 11 Jul 2018 11:21:06 +0100
From:   Will Deacon <will.deacon@....com>
To:     Jiaxun Yang <jiaxun.yang@...goat.com>
Cc:     linux-mips@...ux-mips.org, Peter Zijlstra <peterz@...radead.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 06:05:51PM +0800, Jiaxun Yang wrote:
> On 2018-7-10 Tue at 20:17:27，Peter Zijlstra Wrote：
> 
> Hi Peter
> Since Huacai unable to send email via client, I'm going to reply for him 
>  
> > Sure.. we all got that far. And no, this isn't the _real_ problem. This
> > is a manifestation of the problem.
> > 
> > The problem is that your SFB is broken (per the Linux requirements). We
> > require that stores will become visible. That is, they must not
> > indefinitely (for whatever reason) stay in the store buffer.
> > 
> > > I don't think this is a hardware bug, in design, SFB will flushed to
> > > L1 cache in three cases:
> > > 
> > > 1, data in SFB is full (be a complete cache line);
> > > 2, there is a subsequent read access in the same cache line;
> > > 3, a 'sync' instruction is executed.
> > 
> > And I think this _is_ a hardware bug. You just designed the bug instead
> > of it being by accident.
> Yes, we understood that this hardware feature is not supported by LKML,
> so it should be a hardware bug for LKML.
> > 
> > It doesn't happen an _any_ other architecture except that dodgy
> > ARM11MPCore part. Linux hard relies on stores to become available
> > _eventually_.
> > 
> > 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 ;~)

Will