[v3,2/7] time/sched_clock: Use raw_read_seqcount_latch()

Commit Message

Leo Yan July 16, 2020, 5:11 a.m. UTC

From: "Ahmed S. Darwish" <[e-mail]>

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/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
References: 1809bfa44e10 ("timers, sched/clock: Avoid deadlock during read from NMI")
Signed-off-by: Ahmed S. Darwish <[e-mail]>
Signed-off-by: Leo Yan <[e-mail]>
---
 kernel/time/sched_clock.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Patch

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);
 }