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



> -----原始邮件-----
> 发件人: "Peter Zijlstra" <peterz@infradead.org>
> 发送时间: 2019-04-25 15:33:48 (星期四)
> 收件人: huangpei@loongson.cn
> 抄送: "Paul Burton" <paul.burton@mips.com>, "stern@rowland.harvard.edu" <stern@rowland.harvard.edu>, "akiyks@gmail.com" <akiyks@gmail.com>, "andrea.parri@amarulasolutions.com" <andrea.parri@amarulasolutions.com>, "boqun.feng@gmail.com" <boqun.feng@gmail.com>, "dlustig@nvidia.com" <dlustig@nvidia.com>, "dhowells@redhat.com" <dhowells@redhat.com>, "j.alglave@ucl.ac.uk" <j.alglave@ucl.ac.uk>, "luc.maranget@inria.fr" <luc.maranget@inria.fr>, "npiggin@gmail.com" <npiggin@gmail.com>, "paulmck@linux.ibm.com" <paulmck@linux.ibm.com>, "will.deacon@arm.com" <will.deacon@arm.com>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "torvalds@linux-foundation.org" <torvalds@linux-foundation.org>, "Huacai Chen" <chenhc@lemote.com>
> 主题: 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@loongson.cn 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 C,P1, P2 on 3 different CPU? I do not know much about LKMM, can explain the test case more explicit?

>
> > In one word, the bug only affect local cpu‘s 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 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().


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