Commit 2a58c527 authored Mar 10, 2016 by Thomas Gleixner

cpu/hotplug: Fix smpboot thread ordering

Commit 931ef163 moved the smpboot thread park/unpark invocation to the
state machine. The move of the unpark invocation was premature as it depends
on work in progress patches.

As a result cpu down can fail, because rcu synchronization in takedown_cpu()
eventually requires a functional softirq thread. I never encountered the
problem in testing, but 0day testing managed to provide a reliable reproducer.

Remove the smpboot_threads_park() call from the state machine for now and put
it back into the original place after the rcu synchronization.

I'm embarrassed as I knew about the dependency and still managed to get it
wrong. Hotplug induced brain melt seems to be the only sensible explanation
for that.

Fixes: 931ef163 "cpu/hotplug: Unpark smpboot threads from the state machine"
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>

parent 5d8eb842

kernel/cpu.c

+3 −2

Original line number	Diff line number	Diff line
		@@ -706,8 +706,9 @@ static int takedown_cpu(unsigned int cpu)
		else
		synchronize_rcu();

		/* Park the hotplug thread */
		/* Park the smpboot threads */
		kthread_park(per_cpu_ptr(&cpuhp_state, cpu)->thread);
		smpboot_park_threads(cpu);

		/*
		* Prevent irq alloc/free while the dying cpu reorganizes the
		@@ -1206,7 +1207,7 @@ static struct cpuhp_step cpuhp_ap_states[] = {
		[CPUHP_AP_SMPBOOT_THREADS] = {
		.name = "smpboot:threads",
		.startup = smpboot_unpark_threads,
		.teardown = smpboot_park_threads,
		.teardown = NULL,
		},
		[CPUHP_AP_NOTIFY_ONLINE] = {
		.name = "notify:online",