Messages in this thread | | | From | Ard Biesheuvel <> | Date | Tue, 7 Sep 2021 18:05:47 +0200 | Subject | Re: [PATCH 2/3] ARM: Move thread_info into task_struct (v7 only) |
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On Tue, 7 Sept 2021 at 17:24, Keith Packard <keithp@keithp.com> wrote: > > 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. >
That is still a bit tricky, given that you now have to swap out the per-CPU offset when you migrate a task, and the generic code is not really set up for that.
> > 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. >
I had a stab at this myself today (long boring day with no Internet connection).
https://android-kvm.googlesource.com/linux/+log/refs/heads/ardb/arm32-ti-in-task
It resembles your code in some places - I suppose we went on the same journey in a sense :-) We'll fix up the credits before this gets resubmitted.
Fixing the per-task stack protector plugin on top of these changes should be trivial but I need a coffee first.
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