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Subject[PATCH 3.2 47/70] vmstat: allocate vmstat_wq before it is used
3.2.76-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Michal Hocko <mhocko@suse.com>

commit 751e5f5c753e8d447bcf89f9e96b9616ac081628 upstream.

kernel test robot has reported the following crash:

BUG: unable to handle kernel NULL pointer dereference at 00000100
IP: [<c1074df6>] __queue_work+0x26/0x390
*pdpt = 0000000000000000 *pde = f000ff53f000ff53 *pde = f000ff53f000ff53
Oops: 0000 [#1] PREEMPT PREEMPT SMP SMP
CPU: 0 PID: 24 Comm: kworker/0:1 Not tainted 4.4.0-rc4-00139-g373ccbe #1
Workqueue: events vmstat_shepherd
task: cb684600 ti: cb7ba000 task.ti: cb7ba000
EIP: 0060:[<c1074df6>] EFLAGS: 00010046 CPU: 0
EIP is at __queue_work+0x26/0x390
EAX: 00000046 EBX: cbb37800 ECX: cbb37800 EDX: 00000000
ESI: 00000000 EDI: 00000000 EBP: cb7bbe68 ESP: cb7bbe38
DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
CR0: 8005003b CR2: 00000100 CR3: 01fd5000 CR4: 000006b0
Stack:
Call Trace:
__queue_delayed_work+0xa1/0x160
queue_delayed_work_on+0x36/0x60
vmstat_shepherd+0xad/0xf0
process_one_work+0x1aa/0x4c0
worker_thread+0x41/0x440
kthread+0xb0/0xd0
ret_from_kernel_thread+0x21/0x40

The reason is that start_shepherd_timer schedules the shepherd work item
which uses vmstat_wq (vmstat_shepherd) before setup_vmstat allocates
that workqueue so if the further initialization takes more than HZ we
might end up scheduling on a NULL vmstat_wq. This is really unlikely
but not impossible.

Fixes: 373ccbe59270 ("mm, vmstat: allow WQ concurrency to discover memory reclaim doesn't make any progress")
Reported-by: kernel test robot <ying.huang@linux.intel.com>
Signed-off-by: Michal Hocko <mhocko@suse.com>
Tested-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2: This precise race condition doesn't exist, but there
is a similar potential race with CPU hotplug. So move the alloc_workqueue()
above register_cpu_notifier().]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
mm/vmstat.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1203,9 +1203,10 @@ static int __init setup_vmstat(void)
#ifdef CONFIG_SMP
int cpu;

+ vmstat_wq = alloc_workqueue("vmstat", WQ_FREEZABLE|WQ_MEM_RECLAIM, 0);
+
register_cpu_notifier(&vmstat_notifier);

- vmstat_wq = alloc_workqueue("vmstat", WQ_FREEZABLE|WQ_MEM_RECLAIM, 0);
for_each_online_cpu(cpu)
start_cpu_timer(cpu);
#endif

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