cpu: Defer smpboot kthread unparking until CPU known to scheduler

Currently, smpboot_unpark_threads() is invoked before the incoming CPU
has been added to the scheduler's runqueue structures.  This might
potentially cause the unparked kthread to run on the wrong CPU, since the
correct CPU isn't fully set up yet.

That causes a sporadic, hard to debug boot crash triggering on some
systems, reported by Borislav Petkov, and bisected down to:

  2a442c9c6453 ("x86: Use common outgoing-CPU-notification code")

This patch places smpboot_unpark_threads() in a CPU hotplug
notifier with priority set so that these kthreads are unparked just after
the CPU has been added to the runqueues.

Reported-and-tested-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 31 insertions, 3 deletions

diff --git a/kernel/cpu.c b/kernel/cpu.c
index d46b4dae0ca0..57858cebd6b5 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -448,6 +448,37 @@ out:
 EXPORT_SYMBOL(cpu_down);
 #endif /*CONFIG_HOTPLUG_CPU*/
 
+/*
+ * Unpark per-CPU smpboot kthreads at CPU-online time.
+ */
+static int smpboot_thread_call(struct notifier_block *nfb,
+                               unsigned long action, void *hcpu)
+{
+        int cpu = (long)hcpu;
+
+        switch (action & ~CPU_TASKS_FROZEN) {
+
+        case CPU_ONLINE:
+                smpboot_unpark_threads(cpu);
+                break;
+
+        default:
+                break;
+        }
+
+        return NOTIFY_OK;
+}
+
+static struct notifier_block smpboot_thread_notifier = {
+        .notifier_call = smpboot_thread_call,
+        .priority = CPU_PRI_SMPBOOT,
+};
+
+void __cpuinit smpboot_thread_init(void)
+{
+        register_cpu_notifier(&smpboot_thread_notifier);
+}
+
 /* Requires cpu_add_remove_lock to be held */
 static int _cpu_up(unsigned int cpu, int tasks_frozen)
 {
@@ -487,9 +518,6 @@ static int _cpu_up(unsigned int cpu, int tasks_frozen)
                 goto out_notify;
         BUG_ON(!cpu_online(cpu));
 
-        /* Wake the per cpu threads */
-        smpboot_unpark_threads(cpu);
-
         /* Now call notifier in preparation. */
         cpu_notify(CPU_ONLINE | mod, hcpu);