Re: [PATCH] arm64: arch_timer: fix the infinite recursion when enable ftrace and erratum workaround

[Mark Rutland]

On Thu, Jul 06, 2017 at 08:49:33PM +0800, Ding Tianhong wrote:
> When enable preempt and ftrace, and perform the following steps, the system will hang:
> mount -t debugfs nodev /sys/kernel/debug/
> cd /sys/kernel/debug/tracing/
> echo function_graph > current_tracer
> 
> This is because the preempt_disable and preempt_enable calls would cause infinite
> recursion for some chips which needs the timer erratum workaround and the system
> will hang, so use the preempt_disable/enable_notrace calls to prevent recursion.

Nit: Please wrap commit messages at 72 characters, unless you have
something that wrapping would destroy.

That aside, this looks sane, but a better expanation would be
worthwhile. What exactly causes the recursion? Does ftrace instrument
preempt_{disable,enable} ?

Can you give an example of the recursion?

Thanks,
Mark.

> 
> Fixes: 6acc71ccac71 ("arm64: arch_timer: Allows a CPU-specific erratum to only affect a subset of CPUs")
> Signed-off-by: Ding Tianhong <dingtianhong@xxxxxxxxxx>
> ---
>  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);				\
>  	}								\
> -- 
> 1.9.0
> 
>