cpu: Remove incorrect BUG_ON

Oleg mentioned that there is no actual guarantee the dying cpu's
migration thread is actually finished running when we get there, so
replace the BUG_ON() with a spinloop waiting for it.

Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 4 insertions, 1 deletions

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 3945066fc01d..cb7a1efa9c2b 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -249,8 +249,11 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
          * The migration_call() CPU_DYING callback will have removed all
          * runnable tasks from the cpu, there's only the idle task left now
          * that the migration thread is done doing the stop_machine thing.
+         *
+         * Wait for the stop thread to go away.
          */
-        BUG_ON(!idle_cpu(cpu));
+        while (!idle_cpu(cpu))
+                cpu_relax();
 
         /* This actually kills the CPU. */
         __cpu_die(cpu);