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

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Date:   Thu, 25 Apr 2019 20:51:17 +0800 (GMT+08:00)
From:   huangpei@...ngson.cn
To:     "Peter Zijlstra" <peterz@...radead.org>
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: Re: Re: [RFC][PATCH 2/5] mips/atomic: Fix
 loongson_llsc_mb() wreckage




> -----原始邮件-----
> 发件人: "Peter Zijlstra" <peterz@...radead.org>
> 发送时间: 2019-04-25 20:26:11 (星期四)
> 收件人: huangpei@...ngson.cn
> 抄送: "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>
> 主题: Re: Re: Re: [RFC][PATCH 2/5] mips/atomic: Fix loongson_llsc_mb() wreckage
> 
> On Thu, Apr 25, 2019 at 07:32:59PM +0800, huangpei@...ngson.cn wrote:
> 
> > > > If it is not LL/SC but other memory access from B on V, A's ll/sc can
> > > > follow the atomic semantics even if A violate the coherence protocol
> > > > in the same situation.
> > > 
> > > *shudder*...
> > > 
> > >   C atomic-set
> > > 
> > >   {
> > > 	  atomic_set(v, 1);
> > >   }
> 
> This is the initial state.
> 
> > 
> > > 
> > >   P1(atomic_t *v)
> > >   {
> > > 	  atomic_add_unless(v, 1, 0);
> > >   }
> > > 
> > >   P2(atomic_t *v)
> > >   {
> > > 	  atomic_set(v, 0);
> > >   }
> > > 
> > >   exists
> > >   (v=2)
> > > 
> > > So that one will still work? (that is, v=2 is forbidden)
> > 
> > you mean C，P1, P2 on 3 different CPU? I do not know much about LKMM, can explain the test case more explicit?
> 
> The 'C' is the language identifier, the 'atomic-set' is the litmus name.
> 
> The unnamed block give the initial conditions.
> 
> Pn blocks give code sequences for CPU n
> 
> The 'exists' clause is evaluated after all Pn blocks are done.
> 
> There's others in this thread that can point you to many papers and
> resources on these litmus test thingies.
> 
> So basically the initial value of @v is set to 1.
> 
> Then CPU-1 does atomic_add_unless(v, 1, 0)
>      CPU-2 does atomic_set(v, 0)
> 
> If CPU1 goes first, it will see 1, which is not 0 and thus add 1 to 1
> and obtains 2. Then CPU2 goes and writes 0, so the exist clause sees
> v==0 and doesn't observe 2.
> 
> The other way around, CPU-2 goes first, writes a 0, then CPU-1 goes and
> observes the 0, finds it matches 0 and doesn't add.  Again, the exist
> clause will find 0 doesn't match 2.
> 
> This all goes unstuck if interleaved like:
> 
> 
> 	CPU-1			CPU-2
> 
> 				xor	t0, t0
> 1:	ll	t0, v
> 	bez	t0, 2f
> 				sw	t0, v
> 	add	t0, t1
> 	sc	t0, v
> 	beqz t0, 1b
> 
> (sorry if I got the MIPS asm wrong; it's not something I normally write)
> 
> And the store-word from CPU-2 doesn't make the SC from CPU-1 fail.
> 

loongson's llsc bug DOES NOT fail this litmus( we will not get V=2)；

only speculative memory access from CPU-1 can "blind" CPU-1(here blind means do ll/sc
 wrong）, this speculative memory access can be observed corrently by CPU2. In this 
case, sw from CPU-2 can get I , which can be observed by CPU-1, and clear llbit，then 
failed sc. 



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