From 2a58c527bb566b7abad96289fa5b973040c33c82 Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Thu, 10 Mar 2016 20:42:08 +0100 Subject: [PATCH] cpu/hotplug: Fix smpboot thread ordering Commit 931ef163309e 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: 931ef163309e "cpu/hotplug: Unpark smpboot threads from the state machine" Reported-by: Fengguang Wu Signed-off-by: Thomas Gleixner Cc: Peter Zijlstra --- kernel/cpu.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/kernel/cpu.c b/kernel/cpu.c index 373e831e0faa..bcee286b7c73 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -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", -- 2.20.1