[PATCH v4 1/5] sched_clock: Match scope of read and write seqcounts

[Daniel Thompson]

Currently the scope of the raw_write_seqcount_begin/end in
sched_clock_register far exceeds the scope of the read section in
sched_clock. This gives the impression of safety during cursory review
but achieves little.

Note that this is likely to be a latent issue at present because
sched_clock_register() is typically called before we enable interrupts,
however the issue does risk bugs being needlessly introduced as the code
evolves.

This patch fixes the problem by increasing the scope of the read locking
performed by sched_clock() to cover all data modified by
sched_clock_register.

We also improve clarity by moving writes to struct clock_data that do
not impact sched_clock() outside of the critical section.

Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
Cc: Russell King <linux@xxxxxxxxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
---
 kernel/time/sched_clock.c | 25 +++++++++++--------------
 1 file changed, 11 insertions(+), 14 deletions(-)

diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 01d2d15aa662..3d21a8719444 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -58,23 +58,21 @@ static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift)
 
 unsigned long long notrace sched_clock(void)
 {
-	u64 epoch_ns;
-	u64 epoch_cyc;
-	u64 cyc;
+	u64 cyc, res;
 	unsigned long seq;
 
-	if (cd.suspended)
-		return cd.epoch_ns;
-
 	do {
 		seq = raw_read_seqcount_begin(&cd.seq);
-		epoch_cyc = cd.epoch_cyc;
-		epoch_ns = cd.epoch_ns;
+
+		res = cd.epoch_ns;
+		if (!cd.suspended) {
+			cyc = read_sched_clock();
+			cyc = (cyc - cd.epoch_cyc) & sched_clock_mask;
+			res += cyc_to_ns(cyc, cd.mult, cd.shift);
+		}
 	} while (read_seqcount_retry(&cd.seq, seq));
 
-	cyc = read_sched_clock();
-	cyc = (cyc - epoch_cyc) & sched_clock_mask;
-	return epoch_ns + cyc_to_ns(cyc, cd.mult, cd.shift);
+	return res;
 }
 
 /*
@@ -124,10 +122,11 @@ void __init sched_clock_register(u64 (*read)(void), int bits,
 	clocks_calc_mult_shift(&new_mult, &new_shift, rate, NSEC_PER_SEC, 3600);
 
 	new_mask = CLOCKSOURCE_MASK(bits);
+	cd.rate = rate;
 
 	/* calculate how many ns until we wrap */
 	wrap = clocks_calc_max_nsecs(new_mult, new_shift, 0, new_mask);
-	new_wrap_kt = ns_to_ktime(wrap - (wrap >> 3));
+	cd.wrap_kt = ns_to_ktime(wrap - (wrap >> 3));
 
 	/* update epoch for new counter and update epoch_ns from old counter*/
 	new_epoch = read();
@@ -138,8 +137,6 @@ void __init sched_clock_register(u64 (*read)(void), int bits,
 	raw_write_seqcount_begin(&cd.seq);
 	read_sched_clock = read;
 	sched_clock_mask = new_mask;
-	cd.rate = rate;
-	cd.wrap_kt = new_wrap_kt;
 	cd.mult = new_mult;
 	cd.shift = new_shift;
 	cd.epoch_cyc = new_epoch;
-- 
2.1.0

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