linux-kernel - Re: Re: [RFC][PATCH 2/5] mips/atomic: Fix loongson_llsc

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Date:   Thu, 25 Apr 2019 11:12:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     huangpei@...ngson.cn
Cc:     Paul Burton <paul.burton@...s.com>,
        "stern@...land.harvard.edu" <stern@...land.harvard.edu>,
        "akiyks@...il.com" <akiyks@...il.com>,
        "andrea.parri@...rulasolutions.com" 
        <andrea.parri@...rulasolutions.com>,
        "boqun.feng@...il.com" <boqun.feng@...il.com>,
        "dlustig@...dia.com" <dlustig@...dia.com>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "j.alglave@....ac.uk" <j.alglave@....ac.uk>,
        "luc.maranget@...ia.fr" <luc.maranget@...ia.fr>,
        "npiggin@...il.com" <npiggin@...il.com>,
        "paulmck@...ux.ibm.com" <paulmck@...ux.ibm.com>,
        "will.deacon@....com" <will.deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        Huacai Chen <chenhc@...ote.com>
Subject: Re: Re: [RFC][PATCH 2/5] mips/atomic: Fix loongson_llsc_mb() wreckage

On Thu, Apr 25, 2019 at 09:33:48AM +0200, Peter Zijlstra wrote:
> > Let me explain the bug more specific:
> > 
> > the bug ONLY matters in following situation:
> > 
> > #. more than one cpu (assume cpu A and B) doing ll/sc on same shared
> > var V
> > 
> > #. speculative memory access from A cause A erroneously succeed sc
> > operation, since the erroneously successful sc operation violate the
> > coherence protocol. (here coherence protocol means the rules that CPU
> > follow to implement ll/sc right)
> > 
> > #. B succeed sc operation too, but this sc operation is right both
> > logically and follow the coherence protocol, and makes A's sc wrong
> > logically since only ONE sc operation can succeed.

> > In one word， the bug only affect local cpu‘s ll/sc operation, and
> > affect MP system.

> > PS:
> > 
> > If local_t is only ll/sc manipulated by current CPU， then no need fix it.
> 
> It _should_ be CPU local, but this was not at all clear from reading the
> original changelog nor the comment with loongson_llsc_mb().

However, if it is a coherence issue, the thing is at the cacheline
level, and there is nothing that says the line isn't shared, just the
one variable isn't.

Ideally there should be minimal false sharing, but....