[PATCH] x86/smp: Fix cpuN startup panic

["Chen, LinX Z" <linx.z.chen@intel.com>]

From: Lin Chen <linx.z.chen@intel.com>

We hit a panic while doing cpu hotplug test.
<0>[  627.982857] Kernel panic - not syncing: smp_callin: CPU1 started up but did not get a callout!
<0>[  627.982864]
<4>[  627.982876] Pid: 0, comm: kworker/0:1 Tainted: G ...
<4>[  627.982883] Call Trace:
<4>[  627.982903]  [<c18f2977>] panic+0x66/0x16c
<4>[  627.982918]  [<c12234cc>] ? default_get_apic_id+0x1c/0x40
<4>[  627.982931]  [<c18ef96d>] start_secondary+0xda/0x252

During BSP bootup AP, it is possible that BSP be preempted before
finishing STARTUP sequence of AP(set cpu_callout_mask) which maybe cause
AP busy wait for it. At present, AP will wait for 2 seconds then panic.

This patch let AP waits until BSP finish the startup sequence and gives
WARNING when BSP is preempted more than 2 seconds.

Signed-off-by: Yanmin Zhang <yanmin_zhang@linux.intel.com>
Signed-off-by: Lin Chen <linx.z.chen@intel.com>
---
  arch/x86/kernel/smpboot.c |   11 ++++++-----
  1 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 7c5a8c3..a9e3379 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -165,19 +165,20 @@ static void __cpuinit smp_callin(void)
  	 * Waiting 2s total for startup (udelay is not yet working)
  	 */
  	timeout = jiffies + 2*HZ;
-	while (time_before(jiffies, timeout)) {
+	while (1) {
  		/*
  		 * Has the boot CPU finished it's STARTUP sequence?
  		 */
  		if (cpumask_test_cpu(cpuid, cpu_callout_mask))
  			break;
  		cpu_relax();
+		if (!time_before(jiffies, timeout)) {
+			WARN(1, "%s: CPU%d started up but did not get a callout!\n",
+					__func__, cpuid);
+			timeout = jiffies + 2*HZ;
+		}
  	}

-	if (!time_before(jiffies, timeout)) {
-		panic("%s: CPU%d started up but did not get a callout!\n",
-		      __func__, cpuid);
-	}

  	/*
  	 * the boot CPU has finished the init stage and is spinning
-- 
1.7.1