linux-kernel - [PATCH v3 2/7] time/sched_clock: Use raw_read_seqcount

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Date:   Thu, 16 Jul 2020 13:11:25 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        "Ben Dooks (Codethink)" <ben.dooks@...ethink.co.uk>,
        Paul Cercueil <paul@...pouillou.net>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Cc:     Leo Yan <leo.yan@...aro.org>
Subject: [PATCH v3 2/7] time/sched_clock: Use raw_read_seqcount_latch()

From: "Ahmed S. Darwish" <a.darwish@...utronix.de>

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>
Signed-off-by: Leo Yan <leo.yan@...aro.org>
---
 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 0acaadc3156c..0deaf4b79fb4 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -70,7 +70,7 @@ static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift)
 
 struct clock_read_data *sched_clock_read_begin(unsigned int *seq)
 {
-	*seq = raw_read_seqcount(&cd.seq);
+	*seq = raw_read_seqcount_latch(&cd.seq);
 	return cd.read_data + (*seq & 1);
 }
 
-- 
2.17.1