clocksource/arm_arch_timer: Use static_branch_enable_cpuslocked()

Use the new static_branch_enable_cpuslocked() function to switch
the workaround static key on the CPU hotplug path.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lkml.kernel.org/r/20170801080257.5056-5-marc.zyngier@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

drivers/clocksource/arm_arch_timer.c

@@ -455,7 +455,11 @@ void arch_timer_enable_workaround(const struct arch_timer_erratum_workaround *wa
 			per_cpu(timer_unstable_counter_workaround, i) = wa;
 	}
 
-	static_branch_enable(&arch_timer_read_ool_enabled);
+	/*
+	 * Use the locked version, as we're called from the CPU
+	 * hotplug framework. Otherwise, we end-up in deadlock-land.
+	 */
+	static_branch_enable_cpuslocked(&arch_timer_read_ool_enabled);
 
 	/*
 	 * Don't use the vdso fastpath if errata require using the