Subject: Re: [PATCH] 8xx: get_mmu_context() for (very) FEW_CONTEXTS and KERNEL_PREEMPT race/starvation issue

From: Marcelo Tosatti <[email-masked]>
Date: Wed, 29 Jun 2005 14:00:20 -0300

Hi!

On Wed, Jun 29, 2005 at 05:25:33PM -0400, Guillaume Autran wrote:
> In that case, what happen when a higher priority task steal the context 
> of the lower priority task after get_mmu_context() but before 
> set_mmu_context() ?
> Then when the lower priority task resumes, its context may no longer be 
> valid...
> Do I get this right ?

Yep... but its OK and expected for the "lower prio task" in question to have
its context invalidated: In this case it will call get_mmu_context() again
and reserve the next one available before executing.

> I'm going to do like this instead of my previous attempt:
> 
>        /* Setup new userspace context */
>        preempt_disable();
>        get_mmu_context(next);
>        set_context(next->context, next->pgd);
>        preempt_enable();
> 
> To make sure we don't loose our context in between.

There should be no need - the window for the race is inside
get_mmu_context().

ie. It should be safe to preempt after setting "next_mm_context".