Re: [PATCH] perf: fix possible divide-by-zero in perf_swevent_overflow()

[DDD <dongdong.deng@windriver.com>]

Peter Zijlstra wrote:
> On Thu, 2010-08-26 at 20:36 +0800, DDD wrote:
>> Peter Zijlstra wrote:
>>> On Thu, 2010-08-26 at 20:07 +0800, Dongdong Deng wrote:
>>>> The event->hw.last_period is possible to zero, thus it will
>>>> cause divide_by_zero later in perf_swevent_set_period().
>>> How can it be zero?
>> When I am running the kgdbts to test the hw_breakpoint_layer with kgdb,
>> I get a call trace as following and this problem is hardly to reproduce.
>>
>> Maybe the root cause was from kgdb/hw_breakpoint_layer,
> 
> Yeah, I think there's a bug in the hw_breakpoint stuff, does something
> like the below fix it?

hello Peter,

Thanks for your patch, but I still could reproduce the problem with your 
patch.


Thanks,
Dongdong




> 
> 
>> but add a checking is good to us and harmless. :-)
> 
> Except that code path is already too bloated and should be reduced not
> added to and conditionals are expensive.
> 
> 
> ---
>  kernel/hw_breakpoint.c |   13 ++++++++++++-
>  1 files changed, 12 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/hw_breakpoint.c b/kernel/hw_breakpoint.c
> index d71a987..f57ebee 100644
> --- a/kernel/hw_breakpoint.c
> +++ b/kernel/hw_breakpoint.c
> @@ -600,9 +600,20 @@ static int __init init_hw_breakpoint(void)
>  }
>  core_initcall(init_hw_breakpoint);
>  
> +static int hw_breakpoint_enable(struct perf_event *event)
> +{
> +	struct hw_perf_event *hwc = &event->hw;
> +
> +	if (hwc->sample_period) {
> +		hwc->last_period = hwc->sample_period;
> +		perf_swevent_set_period(event);
> +	}
> +
> +	return arch_install_hw_breakpoint(event);
> +}
>  
>  struct pmu perf_ops_bp = {
> -	.enable		= arch_install_hw_breakpoint,
> +	.enable		= hw_breakpoint_enable,
>  	.disable	= arch_uninstall_hw_breakpoint,
>  	.read		= hw_breakpoint_pmu_read,
>  };
> 
>