linux-kernel - [tip:timers/urgent] time: Fix ktime_get

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Date:   Sat, 26 Aug 2017 07:10:39 -0700
From:   tip-bot for John Stultz <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     prarit@...hat.com, hpa@...or.com, kevin.brodsky@....com,
        tglx@...utronix.de, mingo@...nel.org, richardcochran@...il.com,
        chris@...is-wilson.co.uk, john.stultz@...aro.org,
        will.deacon@....com, stephen.boyd@...aro.org, mlichvar@...hat.com,
        linux-kernel@...r.kernel.org, danielmentz@...gle.com
Subject: [tip:timers/urgent] time: Fix ktime_get_raw() incorrect base
 accumulation

Commit-ID:  0bcdc0987cce9880436b70836c6a92bb8e744fd1
Gitweb:     http://git.kernel.org/tip/0bcdc0987cce9880436b70836c6a92bb8e744fd1
Author:     John Stultz <john.stultz@...aro.org>
AuthorDate: Fri, 25 Aug 2017 15:57:04 -0700
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Sat, 26 Aug 2017 16:06:12 +0200

time: Fix ktime_get_raw() incorrect base accumulation

In comqit fc6eead7c1e2 ("time: Clean up CLOCK_MONOTONIC_RAW time
handling"), the following code got mistakenly added to the update of the
raw timekeeper:

 /* Update the monotonic raw base */
 seconds = tk->raw_sec;
 nsec = (u32)(tk->tkr_raw.xtime_nsec >> tk->tkr_raw.shift);
 tk->tkr_raw.base = ns_to_ktime(seconds * NSEC_PER_SEC + nsec);

Which adds the raw_sec value and the shifted down raw xtime_nsec to the
base value.

But the read function adds the shifted down tk->tkr_raw.xtime_nsec value
another time, The result of this is that ktime_get_raw() users (which are
all internal users) see the raw time move faster then it should (the rate
at which can vary with the current size of tkr_raw.xtime_nsec), which has
resulted in at least problems with graphics rendering performance.

The change tried to match the monotonic base update logic:

 seconds = (u64)(tk->xtime_sec + tk->wall_to_monotonic.tv_sec);
 nsec = (u32) tk->wall_to_monotonic.tv_nsec;
 tk->tkr_mono.base = ns_to_ktime(seconds * NSEC_PER_SEC + nsec);

Which adds the wall_to_monotonic.tv_nsec value, but not the
tk->tkr_mono.xtime_nsec value to the base.

To fix this, simplify the tkr_raw.base accumulation to only accumulate the
raw_sec portion, and do not include the tkr_raw.xtime_nsec portion, which
will be added at read time.

Fixes: fc6eead7c1e2 ("time: Clean up CLOCK_MONOTONIC_RAW time handling")
Reported-and-tested-by: Chris Wilson <chris@...is-wilson.co.uk>
Signed-off-by: John Stultz <john.stultz@...aro.org>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Prarit Bhargava <prarit@...hat.com>
Cc: Kevin Brodsky <kevin.brodsky@....com>
Cc: Richard Cochran <richardcochran@...il.com>
Cc: Stephen Boyd <stephen.boyd@...aro.org>
Cc: Will Deacon <will.deacon@....com>
Cc: Miroslav Lichvar <mlichvar@...hat.com>
Cc: Daniel Mentz <danielmentz@...gle.com>
Link: http://lkml.kernel.org/r/1503701824-1645-1-git-send-email-john.stultz@linaro.org

---
 kernel/time/timekeeping.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index cedafa0..7e7e61c 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -637,9 +637,7 @@ static inline void tk_update_ktime_data(struct timekeeper *tk)
 	tk->ktime_sec = seconds;
 
 	/* Update the monotonic raw base */
-	seconds = tk->raw_sec;
-	nsec = (u32)(tk->tkr_raw.xtime_nsec >> tk->tkr_raw.shift);
-	tk->tkr_raw.base = ns_to_ktime(seconds * NSEC_PER_SEC + nsec);
+	tk->tkr_raw.base = ns_to_ktime(tk->raw_sec * NSEC_PER_SEC);
 }
 
 /* must hold timekeeper_lock */