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.
Fixes: 6acc71ccac71 ("arm64: arch_timer: Allows a CPU-specific erratum to only affect a subset of CPUs")
Signed-off-by: Ding Tianhong <[email protected]>
---
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
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 <[email protected]>
> ---
> 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
>
>
On 2017/7/6 22:35, Mark Rutland wrote:
> 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.
>
OK
> 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?
>
As the 96b3d28bf4(sched/clock: Prevent tracing recursion in sched_clock_cpu()) said, the
preempt_disable/enable() are traced and this cause trace_clock() users to get the local timer
again, it will cause the infinite 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 <[email protected]>
>> ---
>> 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
>>
>>
>
> .
>