[RFC PATCH 6/6] ARM: implement THREAD_INFO_IN_TASK for uniprocessor systems

[Ard Biesheuvel]

On Fri, 26 Nov 2021 at 23:32, Arnd Bergmann <arnd at arndb.de> wrote:
>
> On Fri, Nov 26, 2021 at 11:10 AM Ard Biesheuvel <ardb at kernel.org> wrote:
> > @@ -1169,7 +1169,7 @@ config CURRENT_POINTER_IN_TPIDRURO
> >
> >  config IRQSTACKS
> >         def_bool y
> > -       depends on GENERIC_IRQ_MULTI_HANDLER && THREAD_INFO_IN_TASK
> > +       depends on GENERIC_IRQ_MULTI_HANDLER
> >         select HAVE_IRQ_EXIT_ON_IRQ_STACK
> >         select HAVE_SOFTIRQ_ON_OWN_STACK
>
> Side note: after this, we might want to investigate finishing off
> GENERIC_IRQ_MULTI_HANDLER for all architectures. The
> currently missing platforms are ARM_SINGLE_ARMV7M,
> ARCH_FOOTBRIDGE, ARCH_IOP32X and ARCH_RPC.
>
> These are a bit tricky (presumably this is why they are not converted
> yet), but it should be possible to do.
>
> >  static inline void set_current(struct task_struct *cur)
> >  {
> > -       if (!IS_ENABLED(CONFIG_CURRENT_POINTER_IN_TPIDRURO))
> > +       if (!IS_ENABLED(CONFIG_CURRENT_POINTER_IN_TPIDRURO) &&
> > +           !(IS_ENABLED(CONFIG_SMP) &&
> > +             IS_ENABLED(CONFIG_SMP_ON_UP) &&
> > +             smp_on_up)) {
>
> I think you can just use is_smp() here to simplify the condition. You might
> need to move the definition to a different header if that causes an #include
> loop.
>

OK

> > @@ -39,16 +67,30 @@ static inline struct task_struct *get_current(void)
> >          * https://github.com/ClangBuiltLinux/linux/issues/1485
> >          */
> >         cur = __builtin_thread_pointer();
> > +#elif defined(CONFIG_CURRENT_POINTER_IN_TPIDRURO) || defined(CONFIG_SMP)
> > +       asm("0: mrc p15, 0, %0, c13, c0, 3                      \n\t"
> > +#ifdef CONFIG_CPU_V6
> > +           "1:                                                 \n\t"
> > +           "   .subsection 1                                   \n\t"
> > +           "2: " LOAD_CURRENT
> > +           "   b       1b                                      \n\t"
> > +           "   .previous                                       \n\t"
> > +           "   .pushsection \".alt.smp.init\", \"a\"           \n\t"
> > +           "   .long   0b - .                                  \n\t"
> > +           "   b       . + (2b - 0b)                           \n\t"
> > +           "   .popsection                                     \n\t"
> > +#endif
>
> You mentioned earlier that this gets ugly with SMP_ON_UP on ARMv6, now
> I see what you meant ;-)
>
> I can see an increasing number of reasons for no longer supporting this
> option. As we recently discussed on IRC, this would affect omap2plus_defconfig,
> imx_v6_v7_defconfig and realview_defconfig, which would all have to drop
> either CPU_V6 or SMP. Since you got it working already, this also seems
> better left as a cleanup for another time once we can build consensus on it,
> but my guess is that at this point the benefits of removing it outweigh those
> of keeping it.
>

Agreed.