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List:       linux-arm-kernel
Subject:    Re: [PATCH 2/3] ARM: Move thread_info into task_struct (v7 only)
From:       Keith Packard <keithp () keithp ! com>
Date:       2021-09-07 15:24:36
Message-ID: 874kawcssr.fsf () keithp ! com
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Ard Biesheuvel <ardb@kernel.org> writes:

> Sure, so it is precisely for that reason that it is better to isolate
> changes that can be isolated.

I'll go ahead and split this apart then; that is how I did development,
after all.

> All the time. 'current' essentially never changes value from the POV
> of code running in task context, so there is usually no reason to care
> about preemption/migration when referring to it. Using per-CPU
> variables is what creates the problem here.

Thanks for helping me -- I just got the wrong model stuck in my head
over the weekend somehow.

If I do have this figured out, we should be able to stick the
per_cpu_offset value in thread_info and use TPIDRPRW to hold 'current'
as code using per_cpu_offset should already be disabling
preemption. That should be an easier change than putting a kernel
pointer in a user-visible register.

> Given that we are already relying on the MP extensions for this
> anyway, I personally think that using another thread ID register to
> carry 'current' is a reasonable approach as well, since it would also
> allow us to get per-task stack protector support into the compiler.
> But I would like to hear from some other folks on cc as well.

That would be awesome; I assume that doesn't require leaving
per_cpu_offset in a thread ID register?

In any case, I'll give my plan a try, and then see about trying your
plan as well so I can compare the complexity of the two solutions.

-- 
-keith

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