Re: [PATCH 2/4] x86: get rid of KERNEL_STACK_OFFSET

Ingo Molnar
2015-02-25T09:45:59Z

* Andy Lutomirski <luto@amacapital.net> wrote:

> - BUG_ON(((current_stack_pointer() ^ this_cpu_read_stable(kernel_stack))
> + BUG_ON(((current_stack_pointer() ^
> + (this_cpu_read_stable(kernel_stack) - 1))
> & ~(THREAD_SIZE - 1)) != 0);
>
> preempt_count_sub(HARDIRQ_OFFSET);
>
> I added that in and applied this patch.

So this is not just slightly buggy, it's fundamentally
wrong as well as it removes the possibility of an RSP value
optimization from the 64-bit path, see my previous mail.

The right solution would be to make SAVE_ARGS recognize
when KERNEL_STACK_OFFSET == args-offset and omit the RSP
fixup in that case, or to simply use a __SAVE_ARGS for the
64-bit path, knowing that RSP has the right value already.

Please also add comments that explain the relationship
between percpu::kernel_stack, KERNEL_STACK_OFFSET and the
64-bit system call entry code.

Also, guys, please slow down a bit and be more careful.
Andy, could you please send me all currently pending entry
bits pending in your tree, because all the in-flight
changes make it hard for me to review patches? Please
(re-)send all patches as well as part of the submission.

Thanks,

Ingo