Patch "timekeeping: Avoid taking lock in NMI path with CONFIG_DEBUG_TIMEKEEPING" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

timekeeping: Avoid taking lock in NMI path with CONFIG_DEBUG_TIMEKEEPING

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
timekeeping-avoid-taking-lock-in-nmi-path-with-config_debug_timekeeping.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: John Stultz <[email protected]>
Date: Tue, 23 Aug 2016 16:08:21 -0700
Subject: timekeeping: Avoid taking lock in NMI path with CONFIG_DEBUG_TIMEKEEPING

From: John Stultz <[email protected]>

commit 27727df240c7cc84f2ba6047c6f18d5addfd25ef upstream.

When I added some extra sanity checking in timekeeping_get_ns() under CONFIG_DEBUG_TIMEKEEPING, I missed that the NMI safe __ktime_get_fast_ns() method was using timekeeping_get_ns().

Thus the locking added to the debug checks broke the NMI-safety of __ktime_get_fast_ns().

This patch open-codes the timekeeping_get_ns() logic for __ktime_get_fast_ns(), so can avoid any deadlocks in NMI.

---
 kernel/time/timekeeping.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

@@ -383,7 +383,10 @@ static __always_inline u64 __ktime_get_f
 	do {
 		seq = raw_read_seqcount_latch(&tkf->seq);
 		tkr = tkf->base + (seq & 0x01);
-		now = ktime_to_ns(tkr->base) + timekeeping_get_ns(tkr);
+		now = ktime_to_ns(tkr->base);
+
+		now += clocksource_delta(tkr->read(tkr->clock),
+					 tkr->cycle_last, tkr->mask);
 	} while (read_seqcount_retry(&tkf->seq, seq));
 
 	return now;

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/0110-ntp-Fix-ADJ_SETOFFSET-being-used-w-ADJ_NANO.patch queue-4.4/timekeeping-avoid-taking-lock-in-nmi-path-with-config_debug_timekeeping.patch queue-4.4/0109-time-Verify-time-values-in-adjtimex-ADJ_SETOFFSET-to.patch queue-4.4/0118-clocksource-Allow-unregistering-the-watchdog.patch queue-4.4/timekeeping-cap-array-access-in-timekeeping_debug.patch