[PATCH v5] x86, cpu-hotplug: fix llc shared map unreleased during cpu hotplug

[Wanpeng Li]

BUG: unable to handle kernel NULL pointer dereference at 0000000000000004
IP: [..] find_busiest_group
PGD 5a9d5067 PUD 13067 PMD 0
Oops: 0000 [#3] SMP
[...]
Call Trace:
load_balance
? _raw_spin_unlock_irqrestore
idle_balance
__schedule
schedule
schedule_timeout
? lock_timer_base
schedule_timeout_uninterruptible
msleep
lock_device_hotplug_sysfs
online_store
dev_attr_store
sysfs_write_file
vfs_write
SyS_write
system_call_fastpath

This bug can be triggered by hot add and remove large number of xen
domain0's vcpus repeatedly.

Last level cache shared map is built during cpu up and build sched domain 
routine takes advantage of it to setup sched domain cpu topology, however, 
llc shared map is unreleased during cpu disable which lead to invalid sched 
domain cpu topology. This patch fix it by release llc shared map correctly
during cpu disable.

Reviewed-by: Toshi Kani <toshi.kani@xxxxxx>
Reviewed-by: Yasuaki Ishimatsu <isimatu.yasuaki@xxxxxxxxxxxxxx>
Tested-by: Linn Crosetto <linn@xxxxxx>
Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxxxxxx>
---
v4 -> v5:
 * add the description when the bug can occur
v3 -> v4:
 * simplify backtrace
v2 -> v3:
 * simplify backtrace 
v1 -> v2:
 * fix subject line

 arch/x86/kernel/smpboot.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 5492798..0134ec7 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1292,6 +1292,9 @@ static void remove_siblinginfo(int cpu)
 
 	for_each_cpu(sibling, cpu_sibling_mask(cpu))
 		cpumask_clear_cpu(cpu, cpu_sibling_mask(sibling));
+	for_each_cpu(sibling, cpu_llc_shared_mask(cpu))
+		cpumask_clear_cpu(cpu, cpu_llc_shared_mask(sibling));
+	cpumask_clear(cpu_llc_shared_mask(cpu));
 	cpumask_clear(cpu_sibling_mask(cpu));
 	cpumask_clear(cpu_core_mask(cpu));
 	c->phys_proc_id = 0;
-- 
1.9.1

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