timekeeping: Don't use seqcount loop in ktime_mono_to_any() on 64-bit systems

ktime_mono_to_any() only fetches the offset inside the loop. This is a
single word on 64-bit CPUs, and seqcount_read_begin() implies a full SMP
barrier.

Use READ_ONCE() to fetch the offset instead of doing a seqcount loop on
64-bit and add the matching WRITE_ONCE()'s to update the offsets in
tk_set_wall_to_mono() and tk_update_sleep_time().

[ tglx: Get rid of the #ifdeffery ]

Signed-off-by: Jeff Layton <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Link: https://lore.kernel.org/all/[email protected]

Author: jtlayton

kernel/time/timekeeping.c

@@ -161,13 +161,15 @@ static void tk_set_wall_to_mono(struct timekeeper *tk, struct timespec64 wtm)
 	WARN_ON_ONCE(tk->offs_real != timespec64_to_ktime(tmp));
 	tk->wall_to_monotonic = wtm;
 	set_normalized_timespec64(&tmp, -wtm.tv_sec, -wtm.tv_nsec);
-	tk->offs_real = timespec64_to_ktime(tmp);
-	tk->offs_tai = ktime_add(tk->offs_real, ktime_set(tk->tai_offset, 0));
+	/* Paired with READ_ONCE() in ktime_mono_to_any() */
+	WRITE_ONCE(tk->offs_real, timespec64_to_ktime(tmp));
+	WRITE_ONCE(tk->offs_tai, ktime_add(tk->offs_real, ktime_set(tk->tai_offset, 0)));
 }
 
 static inline void tk_update_sleep_time(struct timekeeper *tk, ktime_t delta)
 {
-	tk->offs_boot = ktime_add(tk->offs_boot, delta);
+	/* Paired with READ_ONCE() in ktime_mono_to_any() */
+	WRITE_ONCE(tk->offs_boot, ktime_add(tk->offs_boot, delta));
 	/*
 	 * Timespec representation for VDSO update to avoid 64bit division
 	 * on every update.
@@ -930,6 +932,14 @@ ktime_t ktime_mono_to_any(ktime_t tmono, enum tk_offsets offs)
 	unsigned int seq;
 	ktime_t tconv;
 
+	if (IS_ENABLED(CONFIG_64BIT)) {
+		/*
+		 * Paired with WRITE_ONCE()s in tk_set_wall_to_mono() and
+		 * tk_update_sleep_time().
+		 */
+		return ktime_add(tmono, READ_ONCE(*offset));
+	}
+
 	do {
 		seq = read_seqcount_begin(&tk_core.seq);
 		tconv = ktime_add(tmono, *offset);