Re: [PATCH 0/2] jump label: 2.6.38 updates

[Masami Hiramatsu]

(2011/02/16 22:24), Mathieu Desnoyers wrote:
> * Will Newton (will.newton@xxxxxxxxx) wrote:
>> On Wed, Feb 16, 2011 at 12:18 PM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>>> On Wed, 2011-02-16 at 10:15 +0000, Will Newton wrote:
>>>
>>>>> That's some really crippled hardware... it does seem like *any* loads
>>>>> from *any* address updated by an sc would have to be done with ll as
>>>>> well, else they may load stale values. One could work this into
>>>>> atomic_read(), but surely there are other places that are problems.
>>>>
>>>> I think it's actually ok, atomics have arch implemented accessors, as
>>>> do spinlocks and atomic bitops. Those are the only place we do sc so
>>>> we can make sure we always ll or invalidate manually.
>>>
>>> I'm curious, how is cmpxchg() implemented on this architecture? As there
>>> are several places in the kernel that uses this on regular variables
>>> without any "accessor" functions.
>>
>> We can invalidate the cache manually. The current cpu will see the new
>> value (post-cache invalidate) and the other cpus will see either the
>> old value or the new value depending on whether they read before or
>> after the invalidate, which is racy but I don't think it is
>> problematic. Unless I'm missing something...
> 
> Assuming the invalidate is specific to a cache-line, I'm concerned about
> the failure of a scenario like the following:
> 
> initially:
> foo = 0
> bar = 0
> 
> CPU A                            CPU B
> 
> xchg(&foo, 1);
>   ll foo
>   sc foo
> 
>   -> interrupt
> 
>   if (foo == 1)
>     xchg(&bar, 1);
>       ll bar
>       sc bar
>       invalidate bar
> 
>                                  lbar = bar;
>                                  smp_mb()
>                                  lfoo = foo;
>                                  BUG_ON(lbar == 1 && lfoo == 0);
>   invalidate foo
> 
> It should be valid to expect that every time "bar" read by CPU B is 1,
> then "foo" is always worth 1. However, in this case, the lack of
> invalidate on foo is keeping the cacheline from reaching CPU B. There
> seems to be a problem with interrupts/NMIs coming right between sc and
> invalidate, as Ingo pointed out.

Hmm, I think that is miss-coding ll/sc.
If I understand correctly, usually cache invalidation should be done
right before storing value, as MSI protocol does.
(or, sc should atomically invalidate the cache line)

Thank you,

-- 
Masami HIRAMATSU
2nd Dept. Linux Technology Center
Hitachi, Ltd., Systems Development Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx
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