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 <pra...@redhat.com>
Cc: Daniel Bristot de Oliveira <bris...@redhat.com>
Cc: Richard Cochran <richardcoch...@gmail.com>
Cc: Jan Kara <j...@suse.cz>
Cc: Jiri Bohac <jbo...@suse.cz>
Cc: Ingo Molnar <mi...@kernel.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Reported-by: Prarit Bhargava <pra...@redhat.com>
Tested-by: Prarit Bhargava <pra...@redhat.com>
Tested-by: Daniel Bristot de Oliveira <bris...@redhat.com>
Signed-off-by: John Stultz <john.stu...@linaro.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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