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SubjectRe: [PATCH 06/14] MIPS: entry: Remove unneeded need_resched() loop
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Hi Valentin,

On Mon, Mar 11, 2019 at 10:47:44PM +0000, Valentin Schneider wrote:
> Since the enabling and disabling of IRQs within preempt_schedule_irq()
> is contained in a need_resched() loop, we don't need the outer arch
> code loop.
>
> Do note that commit a18815abcdfd ("Use preempt_schedule_irq.")
> initially removed the existing loop, but commit cdaed73afb61 ("Fix
> preemption bug.") reintroduced it. It is however not clear what the
> issue was and why such a loop was reintroduced, so I'm probably
> missing something.

It looks to me like commit a18815abcdfd ("Use preempt_schedule_irq.")
forgot the branch to restore_all, so would have fallen through to
ret_from_fork() & done weird things.

Adding the branch to restore_all as you're doing here would have been a
better fix than commit cdaed73afb61 ("Fix preemption bug.").

> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> Cc: Ralf Baechle <ralf@linux-mips.org>
> Cc: Paul Burton <paul.burton@mips.com>
> Cc: James Hogan <jhogan@kernel.org>
> Cc: linux-mips@vger.kernel.org
> ---
> arch/mips/kernel/entry.S | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/arch/mips/kernel/entry.S b/arch/mips/kernel/entry.S
> index d7de8adcfcc8..2240faeda62a 100644
> --- a/arch/mips/kernel/entry.S
> +++ b/arch/mips/kernel/entry.S
> @@ -58,7 +58,6 @@ resume_kernel:
> local_irq_disable
> lw t0, TI_PRE_COUNT($28)
> bnez t0, restore_all
> -need_resched:
> LONG_L t0, TI_FLAGS($28)
> andi t1, t0, _TIF_NEED_RESCHED
> beqz t1, restore_all
> @@ -66,7 +65,7 @@ need_resched:
> andi t0, 1
> beqz t0, restore_all
> jal preempt_schedule_irq
> - b need_resched
> + j restore_all

One nit - why change from branch to jump? It's not a big deal, but I'd
prefer we stick with the branch ("b") instruction for a few reasons:

- restore_all is nearby so there's no issue with it being out of range
of a branch in any variation of the MIPS ISA.

- It's more consistent with the future of the MIPS architecture, eg.
nanoMIPS in which branch instructions all use PC-relative immediate
offsets & jump instructions are always of the "register" variety where
the destination is specified by a register rather than an immediate
encoded in the instruction (the assembler will fix it up & emit a
branch anyway, but I generally prefer to invoke less magic in these
areas...).
- A PC-relative branch won't generate an extra reloc in a relocatable
kernel, whereas a jump will.

Even better would be if we take advantage of this being a tail call & do
this:

PTR_LA ra, restore_all
j preempt_schedule_irq

(Where I left the call to preempt_schedule_irq using a jump because it
may be further away.)

Though I don't mind if you wanna just s/j/b/ & leave the tail call
optimisation for someone else to do as a later change.

Thanks,
Paul

> #endif
>
> FEXPORT(ret_from_kernel_thread)
> --
> 2.20.1
>

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