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

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Date:   Thu, 25 Apr 2019 15:31:05 +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: Re: Re: [RFC][PATCH 2/5] mips/atomic: Fix loongson_llsc_mb()
 wreckage

On Thu, Apr 25, 2019 at 08:51:17PM +0800, huangpei@...ngson.cn wrote:

> > 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. 

I'm not following, suppose CPU-1 happens as a speculation (imagine
whatever code is required to make that happen before). CPU-2 sw will
cause I on CPU-1's ll but, as in the previous email, CPU-1 will continue
as if it still has E and complete the SC.

That is; I'm just not seeing why this case would be different from two
competing LL/SCs.