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: Thu, 25 Apr 2019 09:33:48 +0200 [thread overview]
Message-ID: <20190425073348.GV11158@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <2b2b07cc.bf42.16a52dc4e4d.Coremail.huangpei@loongson.cn>
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)
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?
> 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)
> 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.
> 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().
next prev parent reply other threads:[~2019-04-25 7:34 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 [this message]
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
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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