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

[Will Newton]

On Wed, Feb 16, 2011 at 10:51 PM, Will Simoneau <simoneau@xxxxxxxxxxx> wrote:
> On 12:41 Wed 16 Feb     , Will Newton wrote:
>> On Wed, Feb 16, 2011 at 12:18 PM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>> > 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...
>
> If I understand this correctly, the manual invalidates must propagate to
> all CPUs that potentially read the value, even if there is no
> contention. Doesn't this involve IPIs? How does it not suck?

The cache is shared between cores (in that regard it's more like a
hyper-threaded core than a true multi-core) so is completely coherent,
so this is the one bit that doesn't really suck! Having spoken to our
hardware guys I'm confident that we'll only ever build a handful of
chip designs with the current way of doing ll/sc and hopefully future
cores will do this the "right" way.
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