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SubjectRe: [PATCH] arm64: arch_timer: fix the infinite recursion when enable ftrace and erratum workaround
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@huawei.com>
> ---
> 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
>
>

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