timekeeping: Avoid taking lock in NMI path with CONFIG_DEBUG_TIMEKEEPING

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.

Fixes: 4ca22c2648f9 "timekeeping: Add warnings when overflows or underflows are observed"
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Cc: stable <stable@vger.kernel.org>
Link: http://lkml.kernel.org/r/1471993702-29148-2-git-send-email-john.stultz@linaro.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 3b65746..e07fb09 100644 (file)
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -401,7 +401,10 @@ static __always_inline u64 __ktime_get_fast_ns(struct tk_fast *tkf)
        do {
                seq = raw_read_seqcount_latch(&tkf->seq);
                tkr = tkf->base + (seq & 0x01);
        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;
        } while (read_seqcount_retry(&tkf->seq, seq));
 
        return now;