linux-kernel - [tip: locking/core] time/sched_clock: Use raw_read_seqcount

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Date:   Thu, 10 Sep 2020 15:08:29 -0000
From:   "tip-bot2 for Ahmed S. Darwish" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        x86 <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: [tip: locking/core] time/sched_clock: Use raw_read_seqcount_latch()
 during suspend

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     58faf20a086bd34f91983609e26eac3d5fe76be3
Gitweb:        https://git.kernel.org/tip/58faf20a086bd34f91983609e26eac3d5fe76be3
Author:        Ahmed S. Darwish <a.darwish@...utronix.de>
AuthorDate:    Thu, 27 Aug 2020 13:40:37 +02:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Thu, 10 Sep 2020 11:19:28 +02:00

time/sched_clock: Use raw_read_seqcount_latch() during suspend

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 has one read memory
barrier less than the currently used raw_read_seqcount() API.

Use raw_read_seqcount_latch() for the suspend path.

Commit aadd6e5caaac ("time/sched_clock: Use raw_read_seqcount_latch()")
missed changing that instance of raw_read_seqcount().

References: 1809bfa44e10 ("timers, sched/clock: Avoid deadlock during read from NMI")
Signed-off-by: Ahmed S. Darwish <a.darwish@...utronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20200715092345.GA231464@debian-buster-darwi.lab.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 1c03eec..8c6b5fe 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -258,7 +258,7 @@ void __init generic_sched_clock_init(void)
  */
 static u64 notrace suspended_sched_clock_read(void)
 {
-	unsigned int seq = raw_read_seqcount(&cd.seq);
+	unsigned int seq = raw_read_seqcount_latch(&cd.seq);

 	return cd.read_data[seq & 1].epoch_cyc;
 }