On Mon 23-04-18 19:09:51, Tetsuo Handa wrote:

On 2018/04/20 1:05, syzbot wrote:

quoted

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] SMP KASAN
Dumping ftrace buffer:
�� (ftrace buffer empty)
Modules linked in:
CPU: 0 PID: 28 Comm: kworker/u4:2 Not tainted 4.16.0-rc7+ #368
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: writeback wb_workfn
RIP: 0010:dev_name include/linux/device.h:981 [inline]
RIP: 0010:wb_workfn+0x1a2/0x16b0 fs/fs-writeback.c:1936
RSP: 0018:ffff8801d951f038 EFLAGS: 00010206
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffffffff81bf6ea5
RDX: 000000000000000a RSI: ffffffff87b44840 RDI: 0000000000000050
RBP: ffff8801d951f558 R08: 1ffff1003b2a3def R09: 0000000000000004
R10: ffff8801d951f438 R11: 0000000000000004 R12: 0000000000000100
R13: ffff8801baee0dc0 R14: ffff8801d951f530 R15: ffff8801baee10d8
FS:� 0000000000000000(0000) GS:ffff8801db200000(0000) knlGS:0000000000000000
CS:� 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000000047ff80 CR3: 0000000007a22006 CR4: 00000000001626f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
�process_one_work+0xc47/0x1bb0 kernel/workqueue.c:2113
�process_scheduled_works kernel/workqueue.c:2173 [inline]
�worker_thread+0xa4b/0x1990 kernel/workqueue.c:2252
�kthread+0x33c/0x400 kernel/kthread.c:238
�ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:406

This report says that wb->bdi->dev == NULL

  static inline const char *dev_name(const struct device *dev)
  {
    /* Use the init name until the kobject becomes available */
    if (dev->init_name)
      return dev->init_name;
  
    return kobject_name(&dev->kobj);
  }

  void wb_workfn(struct work_struct *work)
  {
  (...snipped...)
     set_worker_desc("flush-%s", dev_name(wb->bdi->dev));
  (...snipped...)
  }

immediately after ioctl(LOOP_CTL_REMOVE) was requested. It is plausible
because ioctl(LOOP_CTL_REMOVE) sets bdi->dev to NULL after returning from
wb_shutdown().

loop_control_ioctl(LOOP_CTL_REMOVE) {
  loop_remove(lo) {
    del_gendisk(lo->lo_disk) {
      bdi_unregister(disk->queue->backing_dev_info) {
        bdi_remove_from_list(bdi);
        wb_shutdown(&bdi->wb);
        cgwb_bdi_unregister(bdi);
        if (bdi->dev) {
          bdi_debug_unregister(bdi);
          device_unregister(bdi->dev);
          bdi->dev = NULL;
        }
      }
    }
  }
}

For some reason wb_shutdown() is not waiting for wb_workfn() to complete
( or something queues again after WB_registered bit was cleared ) ?

Anyway, I think that this is block layer problem rather than fs layer
problem.

Thanks for the analysis. I think I can see where is the problem -
wb_workfn() can requeue the work while wb_shutdown() is running I'll send a
patch shortly.

By the way, I got a newbie question regarding commit 5318ce7d46866e1d ("bdi:
Shutdown writeback on all cgwbs in cgwb_bdi_destroy()"). It uses clear_bit()
to clear WB_shutting_down bit so that threads waiting at wait_on_bit() will
wake up. But clear_bit() itself does not wake up threads, does it? Who wakes
them up (e.g. by calling wake_up_bit()) after clear_bit() was called?

Yeah, that's a bug. Thanks for fixing it.

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR