# git rev-parse -q --verify 751e5f5c753e8d447bcf89f9e96b9616ac081628^{commit}
751e5f5c753e8d447bcf89f9e96b9616ac081628
already have revision, skipping fetch
# git checkout -q -f -B kisskb 751e5f5c753e8d447bcf89f9e96b9616ac081628
# git clean -qxdf
# git log -1
commit 751e5f5c753e8d447bcf89f9e96b9616ac081628
Author: Michal Hocko <mhocko@suse.com>
Date:   Fri Jan 8 11:18:29 2016 +0100

    vmstat: allocate vmstat_wq before it is used
    
    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: stable@vger.kernel.org
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
# < /opt/cross/gcc-4.6.3-nolibc/um-x86_64-linux/bin/x86_64-linux-gcc --version
# < git log --format=%s --max-count=1 751e5f5c753e8d447bcf89f9e96b9616ac081628
# < make -j 48 ARCH=um O=/home/kisskb/slave/build/linus_um-defconfig_um-x86_64 CROSS_COMPILE=/opt/cross/gcc-4.6.3-nolibc/um-x86_64-linux/bin/x86_64-linux- SUBARCH=x86_64 defconfig
kernel/time/Kconfig:155:warning: range is invalid
# make -j 48 ARCH=um O=/home/kisskb/slave/build/linus_um-defconfig_um-x86_64 CROSS_COMPILE=/opt/cross/gcc-4.6.3-nolibc/um-x86_64-linux/bin/x86_64-linux- SUBARCH=x86_64 
make[1]: Entering directory '/home/kisskb/slave/build/linus_um-defconfig_um-x86_64'
  GEN     ./Makefile
scripts/kconfig/conf  --silentoldconfig arch/x86/um/Kconfig
kernel/time/Kconfig:155:warning: range is invalid
  CHK     include/generated/uapi/linux/version.h
  UPD     include/generated/uapi/linux/version.h
  HOSTCC  scripts/basic/bin2c
  WRAP    arch/um/include/generated/asm/barrier.h
  WRAP    arch/um/include/generated/asm/bug.h
  WRAP    arch/um/include/generated/asm/clkdev.h
  WRAP    arch/um/include/generated/asm/cputime.h
  WRAP    arch/um/include/generated/asm/current.h
  WRAP    arch/um/include/generated/asm/delay.h
  WRAP    arch/um/include/generated/asm/device.h
  WRAP    arch/um/include/generated/asm/emergency-restart.h
  WRAP    arch/um/include/generated/asm/exec.h
  WRAP    arch/um/include/generated/asm/ftrace.h
  WRAP    arch/um/include/generated/asm/futex.h
  WRAP    arch/um/include/generated/asm/hardirq.h
  WRAP    arch/um/include/generated/asm/hw_irq.h
  WRAP    arch/um/include/generated/asm/io.h
  WRAP    arch/um/include/generated/asm/irq_regs.h
  WRAP    arch/um/include/generated/asm/irq_work.h
  WRAP    arch/um/include/generated/asm/kdebug.h
  WRAP    arch/um/include/generated/asm/mcs_spinlock.h
  WRAP    arch/um/include/generated/asm/mm-arch-hooks.h
  WRAP    arch/um/include/generated/asm/mutex.h
  WRAP    arch/um/include/generated/asm/param.h
  WRAP    arch/um/include/generated/asm/pci.h
  WRAP    arch/um/include/generated/asm/percpu.h
  WRAP    arch/um/include/generated/asm/preempt.h
  WRAP    arch/um/include/generated/asm/switch_to.h
  WRAP    arch/um/include/generated/asm/topology.h
  WRAP    arch/um/include/generated/asm/trace_clock.h
  WRAP    arch/um/include/generated/asm/word-at-a-time.h
  WRAP    arch/um/include/generated/asm/xor.h
  CHK     include/config/kernel.release
  GEN     ./Makefile
  HOSTCC  scripts/kallsyms
  CC      scripts/mod/empty.o
  HOSTCC  scripts/mod/mk_elfconfig
  CC      scripts/mod/devicetable-offsets.s
  GEN     scripts/mod/devicetable-offsets.h
  MKELF   scripts/mod/elfconfig.h
  HOSTCC  scripts/mod/modpost.o
  HOSTCC  scripts/mod/file2alias.o
  HOSTCC  scripts/mod/sumversion.o
  UPD     include/config/kernel.release
  SYSTBL  arch/x86/entry/syscalls/../../include/generated/asm/syscalls_32.h
  SYSHDR  arch/x86/entry/syscalls/../../include/generated/asm/unistd_32_ia32.h
  SYSHDR  arch/x86/entry/syscalls/../../include/generated/asm/unistd_64_x32.h
  SYSHDR  arch/x86/entry/syscalls/../../include/generated/uapi/asm/unistd_32.h
  SYSHDR  arch/x86/entry/syscalls/../../include/generated/uapi/asm/unistd_x32.h
  SYSTBL  arch/x86/entry/syscalls/../../include/generated/asm/syscalls_64.h
  SYSHDR  arch/x86/entry/syscalls/../../include/generated/uapi/asm/unistd_64.h
  Using /home/kisskb/slave/src as source for kernel
  CHK     include/generated/utsrelease.h
  UPD     include/generated/utsrelease.h
  HOSTLD  scripts/mod/modpost
  HOSTCC  scripts/unifdef
  CC      arch/x86/um/user-offsets.s
In file included from /usr/include/features.h:374:0,
                 from /usr/include/stdio.h:27,
                 from /home/kisskb/slave/src/arch/x86/um/user-offsets.c:1:
/opt/cross/gcc-4.6.3-nolibc/x86_64-linux/lib/gcc/x86_64-linux/4.6.3/../../../../x86_64-linux/include/sys/cdefs.h:385:27: fatal error: bits/wordsize.h: No such file or directory
compilation terminated.
/home/kisskb/slave/src/scripts/Makefile.build:153: recipe for target 'arch/x86/um/user-offsets.s' failed
make[2]: *** [arch/x86/um/user-offsets.s] Error 1
arch/um/Makefile:150: recipe for target 'arch/x86/um/user-offsets.s' failed
make[1]: *** [arch/x86/um/user-offsets.s] Error 2
make[1]: Leaving directory '/home/kisskb/slave/build/linus_um-defconfig_um-x86_64'
Makefile:146: recipe for target 'sub-make' failed
make: *** [sub-make] Error 2
Command 'make -j 48 ARCH=um O=/home/kisskb/slave/build/linus_um-defconfig_um-x86_64 CROSS_COMPILE=/opt/cross/gcc-4.6.3-nolibc/um-x86_64-linux/bin/x86_64-linux- SUBARCH=x86_64 ' returned non-zero exit status 2
# rm -rf /home/kisskb/slave/build/linus_um-defconfig_um-x86_64
# Build took: 0:00:04.406778