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From: Peter Zijlstra <peterz@infradead.org>
To: Leo Yan <leo.yan@linaro.org>
Cc: "Ahmed S. Darwish" <a.darwish@linutronix.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Will Deacon <will@kernel.org>,
	John Ogness <jogness@linutronix.de>,
	"Sebastian A. Siewior" <bigeasy@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] time/sched_clock: Use raw_read_seqcount_latch()
Date: Wed, 15 Jul 2020 17:58:50 +0200
Message-ID: <20200715155850.GL10769@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200715152926.GA16686@leoy-ThinkPad-X240s>

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@linutronix.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.

  reply index

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-15  2:05 [PATCH v2 0/6] arm64: perf: Proper cap_user_time* support Leo Yan
2020-07-15  2:05 ` [PATCH v2 1/6] sched_clock: Expose struct clock_read_data Leo Yan
2020-07-15  5:56   ` Ahmed S. Darwish
2020-07-15  6:54     ` Leo Yan
2020-07-15  7:21       ` Ahmed S. Darwish
2020-07-15  8:12     ` Peter Zijlstra
2020-07-15  8:14       ` peterz
2020-07-15  9:23         ` Ahmed S. Darwish
2020-07-15  9:52           ` Peter Zijlstra
2020-07-15 11:59             ` [PATCH] time/sched_clock: Use raw_read_seqcount_latch() Ahmed S. Darwish
2020-07-15 15:29               ` Leo Yan
2020-07-15 15:58                 ` Peter Zijlstra [this message]
2020-07-16  5:22                   ` Leo Yan
2020-09-10 15:08           ` [tip: locking/core] time/sched_clock: Use raw_read_seqcount_latch() during suspend tip-bot2 for Ahmed S. Darwish
2020-07-15  2:05 ` [PATCH v2 2/6] arm64: perf: Implement correct cap_user_time Leo Yan
2020-07-15  8:38   ` Peter Zijlstra
2020-07-15 15:39     ` Leo Yan
2020-07-15  2:05 ` [PATCH v2 3/6] arm64: perf: Only advertise cap_user_time for arch_timer Leo Yan
2020-07-15  2:05 ` [PATCH v2 4/6] perf: Add perf_event_mmap_page::cap_user_time_short ABI Leo Yan
2020-07-15  2:05 ` [PATCH v2 5/6] arm64: perf: Add cap_user_time_short Leo Yan
2020-07-15  2:05 ` [PATCH v2 6/6] tools headers UAPI: Update tools's copy of linux/perf_event.h Leo Yan
2020-07-15  5:17 ` [PATCH v2 0/6] arm64: perf: Proper cap_user_time* support Ahmed S. Darwish
2020-07-15  6:29   ` Leo Yan

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