Re: [PATCH] PREEMPT_RT_FULL: ARM context switch needs IRQs enabled

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2011-12-16 at 11:43 +0000, Catalin Marinas wrote:
> From 26d87e955f089fd246bee29bb388f22da1297e0c Mon Sep 17 00:00:00 2001
> From: Catalin Marinas <catalin.marinas@arm.com>
> Date: Fri, 16 Dec 2011 11:32:26 +0000
> Subject: [PATCH] sched, mm: Use activate_mm() instead of switch_mm()
> 
> The ARM port tries to remove __ARCH_WANT_INTERRUPTS_ON_CTXSW. Since the
> actual pgd switching requires interrupts to be enabled on ARM (for
> latency on ARMv5 and earlier and IPIs on ARMv6+), the solution is to
> defer the pgd switching to a post context switch hook that is run with
> interrupts enabled. There are however two additional direct calls to
> switch_mm() without the additional post-switch hook and ARM would fail
> to set the new pgd.
> 
> This patch changes the switch_mm() call with activate_mm() which ensures
> that the required pgd has been set. The activate_mm() function must be
> called with interrupts enabled.
> 
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Peter Zijlstra <peterz@infradead.org>
> ---
>  kernel/sched.c   |    2 +-
>  mm/mmu_context.c |    2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 7b46a39..3976157 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -6310,7 +6310,7 @@ void idle_task_exit(void)
>         BUG_ON(cpu_online(smp_processor_id()));
>  
>         if (mm != &init_mm)
> -               switch_mm(mm, &init_mm, current);
> +               activate_mm(mm, &init_mm);
>         mmdrop(mm);
>  }
>  
> diff --git a/mm/mmu_context.c b/mm/mmu_context.c
> index cf332bc..4e44ac4 100644
> --- a/mm/mmu_context.c
> +++ b/mm/mmu_context.c
> @@ -32,7 +32,7 @@ void use_mm(struct mm_struct *mm)
>                 tsk->active_mm = mm;
>         }
>         tsk->mm = mm;
> -       switch_mm(active_mm, mm, tsk);
> +       activate_mm(active_mm, mm);
>         task_unlock(tsk);
>  
>         if (active_mm != mm)
> 

alpha, cris, microblaze, powerpc appear to actually use that task
argument of switch_mm(), so I'm not convinced this is actually correct.