Re: [PATCH V2] time/sched_clock: mark sched_clock_read_begin/retry as notrace

[Quanyang Wang]

Hi Peter,

On 9/29/20 3:49 PM, Peter Zijlstra wrote:
> On Tue, Sep 29, 2020 at 09:50:51AM +0800, quanyang.wang@xxxxxxxxxxxxx wrote:
> > From: Quanyang Wang <quanyang.wang@xxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxx>
> > ---
> > 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@xxxxxxxxxxxxx>