linux-kernel - Re: [PATCH] x86/preempt: do not clear PREEMPT_NEED

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Date:   Fri, 11 Nov 2016 18:22:03 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/preempt: do not clear PREEMPT_NEED_RESCHED on
 preempt count reset

On Mon, Nov 07, 2016 at 02:01:00PM +0100, Martin Schwidefsky wrote:
> The per-cpu preempt count of x86 contains two values, the actual preempt
> count and the inverted PREEMPT_NEED_RESCHED bit. If a corrupted preempt
> count is detected the preempt_count_set function is used to reset the
> preempt count.
> 
> In case the inverted PREEMPT_NEED_RESCHED bit is zero at the time of the
> reset, the preemption indication is lost. Use raw_cpu_cmpxchg_4 to reset
> only the count part and leave the PREEMPT_NEED_RESCHED bit as it is.
> 
> Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>

Thanks Martin!

I don't suppose this really hurts too much; if we hit a warn where this
restore is needed there are bigger problems, but given you've done the
patch already and it does improve consistency, I've taken it.

> ---
>  arch/x86/include/asm/preempt.h | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/preempt.h b/arch/x86/include/asm/preempt.h
> index 17f2186..ec1f3c6 100644
> --- a/arch/x86/include/asm/preempt.h
> +++ b/arch/x86/include/asm/preempt.h
> @@ -24,7 +24,13 @@ static __always_inline int preempt_count(void)
>  
>  static __always_inline void preempt_count_set(int pc)
>  {
> -	raw_cpu_write_4(__preempt_count, pc);
> +	int old, new;
> +
> +	do {
> +		old = raw_cpu_read_4(__preempt_count);
> +		new = (old & PREEMPT_NEED_RESCHED) |
> +			(pc & ~PREEMPT_NEED_RESCHED);
> +	} while (raw_cpu_cmpxchg_4(__preempt_count, old, new) != old);
>  }
>  
>  /*
> -- 
> 1.9.1
>