Re: [PATCH] sched: Fix 32bit race in sched_clock_remote()

[Steven Rostedt]

On Fri, 2013-04-05 at 18:36 +0200, Peter Zijlstra wrote:
> Thomas spotted a nasty 32bit race in sched_clock_remote() after way too
> many hours of debugging weirdness.
> 
> What happens is that sched_clock_remote() does regular machine word
> reads of sched_clock_data::clock; this appears safe since we use
> cmpxchg64() to update the variable and any half-read value would
> trigger a retry.
> 
> Except we don't validate the new value 'val' in the same way! Thus we
> can propagate non-atomic read errors into the clock value.
> 
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Debugged-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> ---
>  kernel/sched/clock.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
> index c685e31..7042ef7 100644
> --- a/kernel/sched/clock.c
> +++ b/kernel/sched/clock.c
> @@ -170,6 +170,21 @@ static u64 sched_clock_local(struct sched_clock_data *scd)
>  	return clock;
>  }
>  
> +#ifndef CONFIG_64BIT

We need to add a Kconfig:

config 32BIT
	depends on BROKEN

Acked-by: Steven Rostedt <rostedt@xxxxxxxxxxx>

-- Steve

> +/*
> + * 32bit machines can't atomically read a u64 except using cmpxchg64()
> + */
> +static inline u64 scd_read_clock(struct sched_clock_data *scd)
> +{
> +	return cmpxchg64(&scd->clock, 0, 0);
> +}
> +#else
> +static inline u64 scd_read_clock(struct sched_clock_data *scd)
> +{
> +	return scd->clock;
> +}
> +#endif
> +
>  static u64 sched_clock_remote(struct sched_clock_data *scd)
>  {
>  	struct sched_clock_data *my_scd = this_scd();
> @@ -178,8 +193,8 @@ static u64 sched_clock_remote(struct sched_clock_data *scd)
>  
>  	sched_clock_local(my_scd);
>  again:
> -	this_clock = my_scd->clock;
> -	remote_clock = scd->clock;
> +	this_clock = scd_clock_read(my_scd);
> +	remote_clock = scd_clock_read(scd);
>  
>  	/*
>  	 * Use the opportunity that we have both locks
> 


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