x86/smpboot: Unbreak CPU0 hotplug

A hang on CPU0 onlining after a preceding offlining is observed. Trace
shows that CPU0 is stuck in check_tsc_sync_target() waiting for source
CPU to run check_tsc_sync_source() but this never happens. Source CPU,
in its turn, is stuck on synchronize_sched() which is called from
native_cpu_up() -> do_boot_cpu() -> unregister_nmi_handler().

So it's a classic ABBA deadlock, due to the use of synchronize_sched() in
unregister_nmi_handler().

Fix the bug by moving unregister_nmi_handler() from do_boot_cpu() to
native_cpu_up() after cpu onlining is done.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170803105818.9934-1-vkuznets@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 17 insertions, 13 deletions

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index b474c8de7fba..54b9e89d4d6b 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -971,7 +971,8 @@ void common_cpu_up(unsigned int cpu, struct task_struct *idle)
  * Returns zero if CPU booted OK, else error code from
  * ->wakeup_secondary_cpu.
  */
-static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
+static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle,
+                       int *cpu0_nmi_registered)
 {
         volatile u32 *trampoline_status =
                 (volatile u32 *) __va(real_mode_header->trampoline_status);
@@ -979,7 +980,6 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
         unsigned long start_ip = real_mode_header->trampoline_start;
 
         unsigned long boot_error = 0;
-        int cpu0_nmi_registered = 0;
         unsigned long timeout;
 
         idle->thread.sp = (unsigned long)task_pt_regs(idle);
@@ -1035,7 +1035,7 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
                 boot_error = apic->wakeup_secondary_cpu(apicid, start_ip);
         else
                 boot_error = wakeup_cpu_via_init_nmi(cpu, start_ip, apicid,
-                                                     &cpu0_nmi_registered);
+                                                     cpu0_nmi_registered);
 
         if (!boot_error) {
                 /*
@@ -1080,12 +1080,6 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
                  */
                 smpboot_restore_warm_reset_vector();
         }
-        /*
-         * Clean up the nmi handler. Do this after the callin and callout sync
-         * to avoid impact of possible long unregister time.
-         */
-        if (cpu0_nmi_registered)
-                unregister_nmi_handler(NMI_LOCAL, "wake_cpu0");
 
         return boot_error;
 }
@@ -1093,8 +1087,9 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
 int native_cpu_up(unsigned int cpu, struct task_struct *tidle)
 {
         int apicid = apic->cpu_present_to_apicid(cpu);
+        int cpu0_nmi_registered = 0;
         unsigned long flags;
-        int err;
+        int err, ret = 0;
 
         WARN_ON(irqs_disabled());
 
@@ -1131,10 +1126,11 @@ int native_cpu_up(unsigned int cpu, struct task_struct *tidle)
 
         common_cpu_up(cpu, tidle);
 
-        err = do_boot_cpu(apicid, cpu, tidle);
+        err = do_boot_cpu(apicid, cpu, tidle, &cpu0_nmi_registered);
         if (err) {
                 pr_err("do_boot_cpu failed(%d) to wakeup CPU#%u\n", err, cpu);
-                return -EIO;
+                ret = -EIO;
+                goto unreg_nmi;
         }
 
         /*
@@ -1150,7 +1146,15 @@ int native_cpu_up(unsigned int cpu, struct task_struct *tidle)
                 touch_nmi_watchdog();
         }
 
-        return 0;
+unreg_nmi:
+        /*
+         * Clean up the nmi handler. Do this after the callin and callout sync
+         * to avoid impact of possible long unregister time.
+         */
+        if (cpu0_nmi_registered)
+                unregister_nmi_handler(NMI_LOCAL, "wake_cpu0");
+
+        return ret;
 }
 
 /**