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 <schwidef...@de.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
>
