linux-kernel - [PATCH v2] timekeeping: don't use seqcount loop in ktime_mono_to

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Message-Id: <20240910-mgtime-v2-1-e96826ac56f0@kernel.org>
Date: Tue, 10 Sep 2024 08:55:39 -0400
From: Jeff Layton <jlayton@...nel.org>
To: John Stultz <jstultz@...gle.com>, Thomas Gleixner <tglx@...utronix.de>, 
 Stephen Boyd <sboyd@...nel.org>
Cc: Vadim Fedorenko <vadim.fedorenko@...ux.dev>, 
 linux-kernel@...r.kernel.org, Jeff Layton <jlayton@...nel.org>
Subject: [PATCH v2] 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 hosts, and seqcount_read_begin implies a full SMP
barrier.

When BITS_PER_LONG == 64, just do a simple ktime_add instead as there
is no possibility of getting a torn offset value.

Cc: Vadim Fedorenko <vadim.fedorenko@...ux.dev>
Signed-off-by: Jeff Layton <jlayton@...nel.org>
---
Thanks for the review so far, Thomas! Hopefully this looks better.
Disregard my earlier question about making this a static inline too.
That would require making offsets[] a global symbol, which I don't think
we want to do.
---
Changes in v2:
- drop the READ_ONCE
- clean up changelog
- Link to v1: https://lore.kernel.org/r/20240910-mgtime-v1-1-35fb64bd0af5@kernel.org
---
 kernel/time/timekeeping.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 3f524d43d685..a3872f087fbc 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -946,6 +946,14 @@ EXPORT_SYMBOL_GPL(ktime_get_coarse_with_offset_and_floor);
  * @tmono:	time to convert.
  * @offs:	which offset to use
  */
+#if BITS_PER_LONG == 64
+ktime_t ktime_mono_to_any(ktime_t tmono, enum tk_offsets offs)
+{
+	ktime_t *offset = offsets[offs];
+
+	return ktime_add(tmono, *offset);
+}
+#else /* BITS_PER_LONG == 64 */
 ktime_t ktime_mono_to_any(ktime_t tmono, enum tk_offsets offs)
 {
 	ktime_t *offset = offsets[offs];
@@ -959,6 +967,7 @@ ktime_t ktime_mono_to_any(ktime_t tmono, enum tk_offsets offs)
 
 	return tconv;
 }
+#endif /* BITS_PER_LONG == 64 */
 EXPORT_SYMBOL_GPL(ktime_mono_to_any);
 
 /**

---
base-commit: dc8f9bbf6ceb160c7fc5e1a03c5a9318cb61bbd7
change-id: 20240910-mgtime-731eace7cca5

Best regards,
-- 
Jeff Layton <jlayton@...nel.org>