Re: [PATCH] sched: Remove PREEMPT_NEED_RESCHED from generic code

[Alexander Graf]

On 11/28/2013 02:26 PM, Peter Zijlstra wrote:
> Subject: sched: Remove PREEMPT_NEED_RESCHED from generic code
>
> While hunting a preemption issue with Alexander, Ben noticed that the
> currently generic PREEMPT_NEED_RESCHED stuff is horribly broken for
> load-store architectures.
>
> We currently rely on the IPI to fold TIF_NEED_RESCHED into
> PREEMPT_NEED_RESCHED, but when this IPI lands while we already have
> a load for the preempt-count but before the store, the store will erase
> the PREEMPT_NEED_RESCHED change.
>
> The current preempt-count only works on load-store archs because
> interrupts are assumed to be completely balanced wrt their preempt_count
> fiddling; the previous preempt_count load will match the preempt_count
> state after the interrupt and therefore nothing gets lost.
>
> This patch removes the PREEMPT_NEED_RESCHED usage from generic code and
> pushes it into x86 arch code; the generic code goes back to relying on
> TIF_NEED_RESCHED.
>
> Boot tested on x86_64 and compile tested on ppc64.
>
> Reported-by: Alexander Graf <agraf@xxxxxxx>
> Reported-by: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

Fixes the issue for me.

Tested-by: Alexander Graf <agraf@xxxxxxx>


Alex

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