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SubjectRe: 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@infradead.org>
> 发送时间: 2019-04-25 21:31:05 (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: Re: Re: [RFC][PATCH 2/5] mips/atomic: Fix loongson_llsc_mb() wreckage
>
> On Thu, Apr 25, 2019 at 08:51:17PM +0800, huangpei@loongson.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.
>

I get your point. I kept my eye on the sw from CPU-2, but forgot the speculative
mem access from CPU-1.

There is no difference bewteen this one and the former case.

=========================================================================
V = 1

CPU-1 CPU-2

xor t0, t0
1: ll t0, V
beqz t0, 2f

/* if speculative mem
access kick cacheline of
V out, it can blind CPU-1
and make CPU-1 believe it
still hold E on V, and can
NOT see the sw from CPU-2
actually invalid V, which
should clear LLBit of CPU-1,
but not */
sw t0, V // just after sw, V = 0
addiu t0, t0, 1

sc t0, V
/* oops, sc write t0(2)
into V with LLBit */

/* get V=2 */
beqz t0, 1b
nop
2:
================================================================================

if speculative mem access *does not* kick out cache line of V, CPU-1 can see sw
from CPU-2, and clear LLBit, which cause sc fail and retry, That's OK



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