Re: [PATCH v2] arm64: arch_timer: avoid infinite recursion when ftrace is enabled

[Ding Tianhong]

add Danial and Thomas.

On 2017/8/10 10:52, Ding Tianhong wrote:
> On platforms with an arch timer erratum workaround, it's possible for
> arch_timer_reg_read_stable() to recurse into itself when certain
> tracing options are enabled, leading to stack overflows and related
> problems.
> 
> For example, when PREEMPT_TRACER and FUNCTION_GRAPH_TRACER are
> selected, it's possible to trigger this with:
> 
> $ mount -t debugfs nodev /sys/kernel/debug/
> $ echo function_graph > /sys/kernel/debug/tracing/current_tracer
> 
> The problem is that in such cases, preempt_disable() instrumentation
> attempts to acquire a timestamp via trace_clock(), resulting in a call
> back to arch_timer_reg_read_stable(), and hence recursion.
> 
> This patch changes arch_timer_reg_read_stable() to use
> preempt_{disable,enable}_notrace(), which avoids this.
> 
> This problem is similar to the fixed by upstream commit 96b3d28bf4
> ("sched/clock: Prevent tracing recursion in sched_clock_cpu()").
> 
> Fixes: 6acc71ccac71 ("arm64: arch_timer: Allows a CPU-specific erratum to only affect a subset of CPUs")
> Signed-off-by: Ding Tianhong <dingtianhong@xxxxxxxxxx>
> Acked-by: Mark Rutland <mark.rutland@xxxxxxx>
> Acked-by: Marc Zyngier <marc.zyngier@xxxxxxx>
> ---
>  arch/arm64/include/asm/arch_timer.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
> index 74d08e4..67bb7a4 100644
> --- a/arch/arm64/include/asm/arch_timer.h
> +++ b/arch/arm64/include/asm/arch_timer.h
> @@ -65,13 +65,13 @@ struct arch_timer_erratum_workaround {
>  	u64 _val;							\
>  	if (needs_unstable_timer_counter_workaround()) {		\
>  		const struct arch_timer_erratum_workaround *wa;		\
> -		preempt_disable();					\
> +		preempt_disable_notrace();				\
>  		wa = __this_cpu_read(timer_unstable_counter_workaround); \
>  		if (wa && wa->read_##reg)				\
>  			_val = wa->read_##reg();			\
>  		else							\
>  			_val = read_sysreg(reg);			\
> -		preempt_enable();					\
> +		preempt_enable_notrace();				\
>  	} else {							\
>  		_val = read_sysreg(reg);				\
>  	}								\
>