linux-kernel - [PATCH] timekeeping: Copy the shadow-timekeeper over the real timekeeper last

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Date:	Wed, 17 Jun 2015 10:05:53 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	lkml <linux-kernel@...r.kernel.org>
Cc:	John Stultz <john.stultz@...aro.org>,
	Prarit Bhargava <prarit@...hat.com>,
	Daniel Bristot de Oliveira <bristot@...hat.com>,
	Richard Cochran <richardcochran@...il.com>,
	Jan Kara <jack@...e.cz>, Jiri Bohac <jbohac@...e.cz>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH] timekeeping: Copy the shadow-timekeeper over the real timekeeper last

The fix in d151832650ed9 (time: Move clock_was_set_seq update
before updating shadow-timekeeper) was unfortunately incomplete.

The main gist of that change was to do the shadow-copy update
last, so that any state changes were properly duplicated, and
we wouldn't accidentally have stale data in the shadow.

Unfortunately in the main update_wall_time() logic, we update
use the shadow-timekeeper to calculate the next update values,
then while holding the lock, copy the shadow-timekeeper over,
then call timekeeping_update() to do some additional
bookkeeping, (skipping the shadow mirror). The bug with this is
the additional bookkeeping isn't all read-only, and some
changes timkeeper state. Thus we might then overwrite this state
change on the next update.

To avoid this problem, do the timekeeping_update() on the
shadow-timekeeper prior to copying the full state over to
the real-timekeeper.

This avoids problems with both the clock_was_set_seq and
next_leap_ktime being overwritten and possibly the
fast-timekeepers as well.

Many thanks to Prarit for his rigorous testing, which discovered
this problem, along with Prarit and Daniel's work validating this
fix.

Cc: Prarit Bhargava <prarit@...hat.com>
Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
Cc: Richard Cochran <richardcochran@...il.com>
Cc: Jan Kara <jack@...e.cz>
Cc: Jiri Bohac <jbohac@...e.cz>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Reported-by: Prarit Bhargava <prarit@...hat.com>
Tested-by: Prarit Bhargava <prarit@...hat.com>
Tested-by: Daniel Bristot de Oliveira <bristot@...hat.com>
Signed-off-by: John Stultz <john.stultz@...aro.org>
---
(This patch applies to tip/timers/core)

 kernel/time/timekeeping.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 5d67ffb..30b7a40 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1853,8 +1853,9 @@ void update_wall_time(void)
 	 * memcpy under the tk_core.seq against one before we start
 	 * updating.
 	 */
+	timekeeping_update(tk, clock_set);
 	memcpy(real_tk, tk, sizeof(*tk));
-	timekeeping_update(real_tk, clock_set);
+	/* The memcpy must come last. Do not put anything here! */
 	write_seqcount_end(&tk_core.seq);
 out:
 	raw_spin_unlock_irqrestore(&timekeeper_lock, flags);
-- 
1.9.1

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