linux-kernel - Re: [PATCH V2] time/sched_clock: mark sched_clock_read

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Date:   Tue, 29 Sep 2020 16:09:15 +0800
From:   Quanyang Wang <quanyang.wang@...driver.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Leo Yan <leo.yan@...aro.org>, Will Deacon <will@...nel.org>,
        a.darwish@...utronix.de,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Paul Cercueil <paul@...pouillou.net>,
        Randy Dunlap <rdunlap@...radead.org>, ben.dooks@...ethink.co.uk
Subject: Re: [PATCH V2] time/sched_clock: mark sched_clock_read_begin/retry as
 notrace

Hi Peter,

On 9/29/20 3:49 PM, Peter Zijlstra wrote:
> On Tue, Sep 29, 2020 at 09:50:51AM +0800, quanyang.wang@...driver.com wrote:
>> From: Quanyang Wang <quanyang.wang@...driver.com>
>>
>> Since sched_clock_read_begin and sched_clock_read_retry are called
>> by notrace function sched_clock, they shouldn't be traceable either,
>> or else ftrace_graph_caller will run into a dead loop on the path
>> as below (arm for instance):
>>
>>    ftrace_graph_caller
>>      prepare_ftrace_return
>>        function_graph_enter
>>          ftrace_push_return_trace
>>            trace_clock_local
>>              sched_clock
>>                sched_clock_read_begin/retry
>>
>> Fixes: 1b86abc1c645 ("sched_clock: Expose struct clock_read_data")
>> Signed-off-by: Quanyang Wang <quanyang.wang@...driver.com>
>> ---
>> Changes:
>> V2: Add notrace to sched_clock_read_retry according to Peter's suggestion.
>>
>>   kernel/time/sched_clock.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
>> index 1c03eec6ca9b..f629e3f5afbe 100644
>> --- a/kernel/time/sched_clock.c
>> +++ b/kernel/time/sched_clock.c
>> @@ -68,13 +68,13 @@ static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift)
>>   	return (cyc * mult) >> shift;
>>   }
>>   
>> -struct clock_read_data *sched_clock_read_begin(unsigned int *seq)
>> +notrace struct clock_read_data *sched_clock_read_begin(unsigned int *seq)
>>   {
>>   	*seq = raw_read_seqcount_latch(&cd.seq);
>>   	return cd.read_data + (*seq & 1);
>>   }
>>   
>> -int sched_clock_read_retry(unsigned int seq)
>> +int notrace sched_clock_read_retry(unsigned int seq)
>>   {
>>   	return read_seqcount_retry(&cd.seq, seq);
>>   }
> You're inconsistent in the placement of notrace, I prefer the first, the
> second read like notrace is the return type instead of a qualifier.

Thanks for your review. I will send a V3 patch to fix it.

Thanks,

Quanyang

>
> In any case,
>
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>