From: Michal Hocko Date: Fri, 8 Jan 2016 10:18:29 +0000 (+0100) Subject: vmstat: allocate vmstat_wq before it is used X-Git-Tag: v3.2.76~24 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=065bc97edc4bc250ec4d6dcc716c9a3e1e1c602c;p=pandora-kernel.git vmstat: allocate vmstat_wq before it is used commit 751e5f5c753e8d447bcf89f9e96b9616ac081628 upstream. kernel test robot has reported the following crash: BUG: unable to handle kernel NULL pointer dereference at 00000100 IP: [] __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:[] 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 Signed-off-by: Michal Hocko Tested-by: Tetsuo Handa Cc: Andrew Morton Signed-off-by: Linus Torvalds [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 --- diff --git a/mm/vmstat.c b/mm/vmstat.c index 14d8cb491ef7..887e83f2135c 100644 --- 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