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

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Date:   Tue, 14 May 2019 17:58:13 +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


I think this thread got 'lost'

On Thu, Apr 25, 2019 at 11:12:58AM +0200, Peter Zijlstra wrote:
> 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....

So if two variables share a line, and one is local while the other is
shared atomic, can contention on the line, but not the variable, cause
issues for the local variable?

If not; why not? Because so far the issue is line granular due to the
coherence aspect.