Re: Re: Re: [RFC][PATCH 2/5] mips/atomic: Fix loongson_llsc_mb() wreckage

[huangpei]

> -----ååéä-----
> åää: "Peter Zijlstra" <peterz@xxxxxxxxxxxxx>
> åéæé: 2019-04-25 15:33:48 (ææå)
> æää: huangpei@xxxxxxxxxxx
> æé: "Paul Burton" <paul.burton@xxxxxxxx>, "stern@xxxxxxxxxxxxxxxxxxx" <stern@xxxxxxxxxxxxxxxxxxx>, "akiyks@xxxxxxxxx" <akiyks@xxxxxxxxx>, "andrea.parri@xxxxxxxxxxxxxxxxxxxx" <andrea.parri@xxxxxxxxxxxxxxxxxxxx>, "boqun.feng@xxxxxxxxx" <boqun.feng@xxxxxxxxx>, "dlustig@xxxxxxxxxx" <dlustig@xxxxxxxxxx>, "dhowells@xxxxxxxxxx" <dhowells@xxxxxxxxxx>, "j.alglave@xxxxxxxxx" <j.alglave@xxxxxxxxx>, "luc.maranget@xxxxxxxx" <luc.maranget@xxxxxxxx>, "npiggin@xxxxxxxxx" <npiggin@xxxxxxxxx>, "paulmck@xxxxxxxxxxxxx" <paulmck@xxxxxxxxxxxxx>, "will.deacon@xxxxxxx" <will.deacon@xxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>, "torvalds@xxxxxxxxxxxxxxxxxxxx" <torvalds@xxxxxxxxxxxxxxxxxxxx>, "Huacai Chen" <chenhc@xxxxxxxxxx>
> äé: Re: Re: [RFC][PATCH 2/5] mips/atomic: Fix loongson_llsc_mb() wreckage
> 
> 
> A: Because it messes up the order in which people normally read text.
> Q: Why is top-posting such a bad thing?
> A: Top-posting.
> Q: What is the most annoying thing in e-mail?
> 
> On Thu, Apr 25, 2019 at 12:58:50PM +0800, huangpei@xxxxxxxxxxx wrote:
> > In my opinion. patch 2/3 is about Loongson's bug, and patch 4/5 is another theme.
> 
> Agreed; it's just that looking at the MIPS code to fix 4/5 made me trip
> over this stuff.
> 
> > 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.
> 
> (I know your coherence protocol is probably more complicated than MESI,
> but bear with me)

our coherentce protocal is simpler than MESI.
> 
> So A speculatively gets V's line in Exclusive mode, speculates the Lock
> flag is still there and completes the Store. This speculative store then
> leaks out and violates MESI because there _should_ only be one Exclusive
> owner of a line (B).
> 
> Something like that?

Yes

> 
> > 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);
>   }


> 
>   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 CïP1, P2 on 3 different CPU? I do not know much about LKMM, can explain the test case more explicit?

> 
> > In one wordï the bug only affect local cpuâs ll/sc operation, and
> > affect MP system.
> 
> Because it is a coherence issue, triggered by a reorder. OK.

Not exactly, it is a ll/sc issue, triggered by speculative memory access from local cpu
 based on what I was told.
> 
> > PS:
> > 
> > If local_t is only ll/sc manipulated by current CPUï 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().


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