linux-kernel - Re: [PATCH] time/sched_clock: Use raw_read_seqcount

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Date:   Wed, 15 Jul 2020 17:58:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>,
        John Ogness <jogness@...utronix.de>,
        "Sebastian A. Siewior" <bigeasy@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] time/sched_clock: Use raw_read_seqcount_latch()

On Wed, Jul 15, 2020 at 11:29:26PM +0800, Leo Yan wrote:
> Hi Peter, Ahmed,
> 
> On Wed, Jul 15, 2020 at 01:59:01PM +0200, Ahmed S. Darwish wrote:
> > sched_clock uses seqcount_t latching to switch between two storage
> > places protected by the sequence counter. This allows it to have
> > interruptible, NMI-safe, seqcount_t write side critical sections.
> > 
> > Since 7fc26327b756 ("seqlock: Introduce raw_read_seqcount_latch()"),
> > raw_read_seqcount_latch() became the standardized way for seqcount_t
> > latch read paths. Due to the dependent load, it also has one read
> > memory barrier less than the currently used raw_read_seqcount() API.
> > 
> > Use raw_read_seqcount_latch() for the seqcount_t latch read path.
> > 
> > Link: https://lkml.kernel.org/r/20200625085745.GD117543@hirez.programming.kicks-ass.net
> > Link: https://lkml.kernel.org/r/20200715092345.GA231464@debian-buster-darwi.lab.linutronix.de
> > References: 1809bfa44e10 ("timers, sched/clock: Avoid deadlock during read from NMI")
> > Signed-off-by: Ahmed S. Darwish <a.darwish@...utronix.de>
> > ---
> >  kernel/time/sched_clock.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
> > index fa3f800d7d76..ea007928d681 100644
> > --- a/kernel/time/sched_clock.c
> > +++ b/kernel/time/sched_clock.c
> > @@ -100,7 +100,7 @@ unsigned long long notrace sched_clock(void)
> >  	struct clock_read_data *rd;
> > 
> >  	do {
> > -		seq = raw_read_seqcount(&cd.seq);
> > +		seq = raw_read_seqcount_latch(&cd.seq);
> 
> Understand this is doing the same thing with __ktime_get_fast_ns() and
> I saw Peter acked to make change for this.
> 
> Just want to confirm, since this patch introduces conflict with the
> patch set "arm64: perf: Proper cap_user_time* support" [1], I should
> rebase the patch set on top of this patch, right?

Or rebase this patch on top of yours and include it, either way.