From: "Ahmed S. Darwish" <a.darwish@linutronix.de>
To: Leo Yan <leo.yan@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
Will Deacon <will@kernel.org>,
Mark Rutland <mark.rutland@arm.com>,
Ingo Molnar <mingo@redhat.com>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Thomas Gleixner <tglx@linutronix.de>,
Paul Cercueil <paul@crapouillou.net>,
"Ben Dooks (Codethink)" <ben.dooks@codethink.co.uk>,
Adrian Hunter <adrian.hunter@intel.com>,
Kan Liang <kan.liang@linux.intel.com>,
jogness@linutronix.de, linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/6] sched_clock: Expose struct clock_read_data
Date: Wed, 15 Jul 2020 09:21:12 +0200 [thread overview]
Message-ID: <20200715072112.GA231369@debian-buster-darwi.lab.linutronix.de> (raw)
In-Reply-To: <20200715065407.GB19269@leoy-ThinkPad-X240s>
On Wed, Jul 15, 2020 at 02:54:07PM +0800, Leo Yan wrote:
> On Wed, Jul 15, 2020 at 07:56:50AM +0200, Ahmed S. Darwish wrote:
> > On Wed, Jul 15, 2020 at 10:05:07AM +0800, Leo Yan wrote:
> > > From: Peter Zijlstra <peterz@infradead.org>
> > >
> > ...
> > >
> > > Provide struct clock_read_data and two (seqcount) helpers so that
> > > architectures (arm64 in specific) can expose the numbers to userspace.
> > >
> > ...
> > >
> > > +struct clock_read_data *sched_clock_read_begin(unsigned int *seq)
> > > +{
> > > + *seq = raw_read_seqcount(&cd.seq);
> > > + return cd.read_data + (*seq & 1);
> > > +}
> > > +
> > ...
> >
> > Hmm, this seqcount_t is actually a latch seqcount. I know the original
> > code also used raw_read_seqcount(), but while at it, let's use the
> > proper read API for seqcount_t latchers: raw_read_seqcount_latch().
>
> Good point. To be honest, I think myself cannot give a good judgement
> for memory barrier related thing :)
>
> I read a bit the document for the latch technique [1], comparing to
> raw_read_seqcount_latch(), the function raw_read_seqcount() contains
> smp_rmb(), IIUC, the *read* memory barrier is used to support for
> kcsan.
>
The smp_rmb() has no relation whatsoever to KCSAN. It pairs with the
write memory barriers in the seqcount_t write path.
AFAIK, PeterZ is the author of this patch, so let's wait for his input
here.
Thanks,
--
Ahmed S. Darwish
Linutronix GmbH
next prev parent reply other threads:[~2020-07-15 7:21 UTC|newest]
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 [this message]
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
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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