clocksource: Install completely before selecting

commit e05b2efb82596905ebfe88e8612ee81dec9b6592 upstream.

Christian Hoffmann reported that the command line clocksource override
with acpi_pm timer fails:

 Kernel command line: <SNIP> clocksource=acpi_pm
 hpet clockevent registered
 Switching to clocksource hpet
 Override clocksource acpi_pm is not HRT compatible.
 Cannot switch while in HRT/NOHZ mode.

The watchdog code is what enables CLOCK_SOURCE_VALID_FOR_HRES, but we
actually end up selecting the clocksource before we enqueue it into
the watchdog list, so that's why we see the warning and fail to switch
to acpi_pm timer as requested. That's particularly bad when we want to
debug timekeeping related problems in early boot.

Put the selection call last.

Reported-by: Christian Hoffmann <email@christianhoffmann.info>
Signed-off-by: John Stultz <johnstul@us.ibm.com>
Link: http://lkml.kernel.org/r/%3C1304558210.2943.24.camel%40work-vm%3E
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 0e984977f28d827e7f622e1d8c66019a1f2ca2dd..7b26a3a1876d0714f8fe2067273d02386e5b49e3 100644 (file)
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -626,8 +626,8 @@ int clocksource_register(struct clocksource *cs)
 
        mutex_lock(&clocksource_mutex);
        clocksource_enqueue(cs);
-       clocksource_select();
        clocksource_enqueue_watchdog(cs);
+       clocksource_select();
        mutex_unlock(&clocksource_mutex);
        return 0;
 }