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SubjectRe: Jump Label initialization
On 09/29/2011 09:45 AM, Jeremy Fitzhardinge wrote:
> On 09/29/2011 05:04 AM, Jan Glauber wrote:
>>
>> s390 does not have the early() variant since it didn't need it. On
>> pre-SMP we probably don't need stop_machine() so creating
>> arch_jump_label_transform_early() by leaving out stop_machine()
>> and patching the code directly should be fine.
>
> Well, it occurs to me that if you're using jump_label_enable() very
> early, then it will be using the full arch_jump_label_transform()
> anyway. While the stop_machine(), locking, etc that the full version
> does is redundant in a pre-SMP environment, it shouldn't hurt, so
> perhaps there's no need for an _early variant at all.
>

You need a way to guarantee that the code you are modifying is not in
the instruction cache of the off-line CPUs.

On our (Cavium) SMP MIPS systems, these off-line CPUs are most likely
spinning in architecture specific smp startup code somewhere in the
kernel image. If this code happened to be adjacent to something the
arch_jump_label_transform_early() would touch, we could have a problem.

The solution is fairly simple, just invalidate the instruction cache
immediately upon leaving the spinning loop.

David Daney


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