From: Peter Zijlstra <peterz@infradead.org>
To: huangpei@loongson.cn
Cc: 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>
Subject: Re: Re: [RFC][PATCH 2/5] mips/atomic: Fix loongson_llsc_mb() wreckage
Date: Tue, 14 May 2019 17:58:13 +0200 [thread overview]
Message-ID: <20190514155813.GG2677@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20190425091258.GC14281@hirez.programming.kicks-ass.net>
I think this thread got 'lost'
On Thu, Apr 25, 2019 at 11:12:58AM +0200, Peter Zijlstra wrote:
> On Thu, Apr 25, 2019 at 09:33:48AM +0200, Peter Zijlstra wrote:
> > > 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.
>
> > > In one word, the bug only affect local cpu‘s ll/sc operation, and
> > > affect MP system.
>
> > > 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().
>
> However, if it is a coherence issue, the thing is at the cacheline
> level, and there is nothing that says the line isn't shared, just the
> one variable isn't.
>
> Ideally there should be minimal false sharing, but....
So if two variables share a line, and one is local while the other is
shared atomic, can contention on the line, but not the variable, cause
issues for the local variable?
If not; why not? Because so far the issue is line granular due to the
coherence aspect.
next prev parent reply other threads:[~2019-05-14 15:58 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-24 12:36 [RFC][PATCH 0/5] atomic: Fixes to smp_mb__{before,after}_atomic() and mips Peter Zijlstra
2019-04-24 12:36 ` [RFC][PATCH 1/5] mips/atomic: Fix cmpxchg64 barriers Peter Zijlstra
2019-04-24 21:00 ` Paul Burton
2019-04-25 6:59 ` Peter Zijlstra
2019-04-24 12:36 ` [RFC][PATCH 2/5] mips/atomic: Fix loongson_llsc_mb() wreckage Peter Zijlstra
2019-04-24 12:59 ` Peter Zijlstra
2019-04-24 21:18 ` Paul Burton
2019-04-25 4:58 ` huangpei
2019-04-25 7:33 ` Peter Zijlstra
2019-04-25 9:09 ` Peter Zijlstra
2019-04-25 12:14 ` huangpei
2019-04-25 9:12 ` Peter Zijlstra
2019-05-14 15:58 ` Peter Zijlstra [this message]
2019-05-14 16:10 ` Linus Torvalds
2019-05-14 16:56 ` Peter Zijlstra
2019-05-14 17:07 ` Linus Torvalds
2019-05-15 13:50 ` huangpei
2019-04-25 11:32 ` huangpei
2019-04-25 12:26 ` Peter Zijlstra
2019-04-25 12:51 ` huangpei
2019-04-25 13:31 ` Peter Zijlstra
2019-04-26 2:57 ` huangpei
2019-05-14 15:46 ` Peter Zijlstra
2019-04-25 16:12 ` Linus Torvalds
2019-04-25 7:15 ` Peter Zijlstra
2019-04-24 12:36 ` [RFC][PATCH 3/5] mips/atomic: Optimize loongson3_llsc_mb() Peter Zijlstra
2019-04-24 12:37 ` [RFC][PATCH 4/5] mips/atomic: Fix smp_mb__{before,after}_atomic() Peter Zijlstra
2019-04-24 21:24 ` Paul Burton
2019-04-25 7:34 ` Peter Zijlstra
2019-04-24 12:37 ` [RFC][PATCH 5/5] x86/atomic: " Peter Zijlstra
2019-04-24 13:41 ` Will Deacon
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