Subject: Re: [PATCH] powerpc: check_and_cede_processor never cedes

From: Benjamin Herrenschmidt <[email-masked]>
Date: Thu, 28 Jun 2012 17:10:44 +1000

On Thu, 2012-06-28 at 09:13 +1000, Anton Blanchard wrote:
> Hi,
> 
> > I'd rather add a helper, something like lazy_irq_pending()
> > and hide the actual check for the bits in irq_happened, in
> > case we change the scheme again.
> 
> Good idea. Look ok?
> 
> --
> 
> Commit f948501b36c6 ("Make hard_irq_disable() actually hard-disable
> interrupts") caused check_and_cede_processor to stop working.
> ->irq_happened will never be zero right after a hard_irq_disable
> so the compiler removes the call to cede_processor completely.
> 
> The bug was introduced back in the lazy interrupt handling rework
> of 3.4 but was hidden until recently because hard_irq_disable did
> nothing.
> 
> This issue will eventually appear in 3.4 stable since the
> hard_irq_disable fix is marked stable, so mark this one for stable
> too.

Yup, looks good, I'll send to Linus tomorrow.

Cheers,
Ben.

> Signed-off-by: Anton Blanchard <[email-masked]>  
> Cc: [email-masked]
> ---
> 
> v2: create a helper, suggested by Ben.
> 
> Index: linux-build/arch/powerpc/platforms/pseries/processor_idle.c
> ===================================================================
> --- linux-build.orig/arch/powerpc/platforms/pseries/processor_idle.c	2012-06-28 08:55:09.422198154 +1000
> +++ linux-build/arch/powerpc/platforms/pseries/processor_idle.c	2012-06-28 08:57:36.112591023 +1000
> @@ -106,7 +106,7 @@ static void check_and_cede_processor(voi
>  	 * we first hard disable then check.
>  	 */
>  	hard_irq_disable();
> -	if (get_paca()->irq_happened == 0)
> +	if (!lazy_irq_pending())
>  		cede_processor();
>  }
>  
> Index: linux-build/arch/powerpc/include/asm/hw_irq.h
> ===================================================================
> --- linux-build.orig/arch/powerpc/include/asm/hw_irq.h	2012-06-21 09:16:26.265354429 +1000
> +++ linux-build/arch/powerpc/include/asm/hw_irq.h	2012-06-28 08:59:22.082320381 +1000
> @@ -103,6 +103,11 @@ static inline void hard_irq_disable(void
>  /* include/linux/interrupt.h needs hard_irq_disable to be a macro */
>  #define hard_irq_disable	hard_irq_disable
>  
> +static inline bool lazy_irq_pending(void)
> +{
> +	return !!(get_paca()->irq_happened & ~PACA_IRQ_HARD_DIS);
> +}
> +
>  /*
>   * This is called by asynchronous interrupts to conditionally
>   * re-enable hard interrupts when soft-disabled after having