On Mon 23-04-18 19:09:51, Tetsuo Handa wrote: > On 2018/04/20 1:05, syzbot wrote: > > 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 <j...@suse.com> SUSE Labs, CR