FYI, we noticed the following commit (built with gcc-7): commit: b636fd38dc40113f853337a7d2a6885ad23b8811 ("cgroup: Implement css_task_iter_skip()") https://git.kernel.org/cgit/linux/kernel/git/tj/cgroup.git for-next in testcase: xfstests with following parameters: disk: 4HDD fs: ext4 test: generic-slow3 test-description: xfstests is a regression test suite for xfs and other files ystems. test-url: git://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git on test machine: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -smp 2 -m 2G caused below changes (please refer to attached dmesg/kmsg for entire log/backtrace): +--------------------------------------------------+------------+------------+ | | 6b115bf58e | b636fd38dc | +--------------------------------------------------+------------+------------+ | boot_successes | 107 | 252 | | boot_failures | 998 | 941 | | BUG:kernel_reboot-without-warning_in_test_stage | 943 | 862 | | BUG:soft_lockup-CPU##stuck_for#s | 9 | 12 | | RIP:simple_write_begin | 1 | 1 | | Kernel_panic-not_syncing:softlockup:hung_tasks | 9 | 12 | | RIP:__memcpy | 4 | 5 | | BUG:kernel_hang_in_boot_stage | 44 | 55 | | BUG:unable_to_handle_page_fault_for_address | 2 | 1 | | Oops:#[##] | 2 | 9 | | RIP:compaction_alloc | 2 | | | Kernel_panic-not_syncing:Fatal_exception | 2 | 10 | | RIP:inode_init_once | 1 | 1 | | RIP:clear_page_rep | 2 | 4 | | WARNING:at_net/sched/sch_generic.c:#dev_watchdog | 1 | 1 | | RIP:dev_watchdog | 1 | 1 | | RIP:new_slab | 1 | | | RIP:kfree | 0 | 1 | | WARNING:at_kernel/fork.c:#__put_task_struct | 0 | 4 | | RIP:__put_task_struct | 0 | 4 | | BUG:kernel_NULL_pointer_dereference,address | 0 | 8 | | RIP:refcount_dec_not_one | 0 | 4 | | RIP:__task_pid_nr_ns | 0 | 5 | | RIP:copy_page | 0 | 1 | | general_protection_fault:#[##] | 0 | 1 | | RIP:_raw_spin_lock_irqsave | 0 | 1 | | BUG:kernel_hang_in_test_stage | 0 | 1 | | RIP:__add_to_page_cache_locked | 0 | 1 | +--------------------------------------------------+------------+------------+ If you fix the issue, kindly add following tag Reported-by: kernel test robot <rong.a.chen(a)intel.com&gt; [ 180.263004] WARNING: CPU: 0 PID: 1 at kernel/fork.c:721 __put_task_struct+0xf1/0x160 [ 180.271243] Modules linked in: ext4 mbcache jbd2 dm_mod crct10dif_pclmul crc32_pclmul sr_mod crc32c_intel cdrom sg ghash_clmulni_intel ppdev snd_pcm ata_generic pata_acpi snd_timer snd aesni_intel bochs_drm crypto_simd soundcore ttm cryptd ata_piix drm_kms_helper glue_helper libata joydev pcspkr serio_raw syscopyarea sysfillrect sysimgblt fb_sys_fops parport_pc drm i2c_piix4 parport floppy ip_tables [ 180.290674] CPU: 0 PID: 1 Comm: systemd Not tainted 5.2.0-rc2-00012-gb636fd3 #2 [ 180.294853] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014 [ 180.299779] RIP: 0010:__put_task_struct+0xf1/0x160 [ 180.303059] Code: ff 4a 5c 74 40 48 8b 3d 85 52 ea 01 48 89 ee e8 65 70 1e 00 48 89 df e8 6d 90 08 00 85 c0 75 b0 48 89 df 5b 5d e9 bf fe ff ff <0f> 0b 8b 43 20 85 c0 0f 84 25 ff ff ff 0f 0b e9 1e ff ff ff 0f 0b [ 180.313348] RSP: 0018:ffffafec00327d28 EFLAGS: 00010246 [ 180.317164] RAX: 0000000000000000 RBX: ffff98d0fed705b0 RCX: 0000000000000000 [ 180.321590] RDX: ffffffff8445ef60 RSI: 0000000000000001 RDI: ffff98d0fed705b0 [ 180.326024] RBP: 0000000000000000 R08: ffff98d0fed712a0 R09: ffff98d08fc03c80 [ 180.330338] R10: ffff98d08fc02c00 R11: 0000000000000000 R12: 0000000000000000 [ 180.334917] R13: ffff98d0f67d0000 R14: ffff98d0f67d0000 R15: 0000000000000001 [ 180.339418] FS: 00007f077dded500(0000) GS:ffff98d0eb000000(0000) knlGS:0000000000000000 [ 180.344717] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 180.348466] CR2: 00007fed5b19b5d8 CR3: 000000007f20c000 CR4: 00000000000006f0 [ 180.352873] Call Trace: [ 180.357757] css_task_iter_next+0x82/0x90 [ 180.361257] pidlist_array_load+0xa9/0x2c0 [ 180.364362] cgroup_pidlist_start+0xe9/0x120 [ 180.367408] kernfs_seq_start+0x4d/0x90 [ 180.370345] seq_read+0xa9/0x3d0 [ 180.373048] vfs_read+0x9b/0x150 [ 180.375795] ksys_read+0xa1/0xe0 [ 180.377689] do_syscall_64+0x5b/0x1e0 [ 180.380360] entry_SYSCALL_64_after_hwframe+0x44/0xa9 [ 180.383052] RIP: 0033:0x7f077c3b8910 [ 180.385124] Code: b6 fe ff ff 48 8d 3d 0f be 08 00 48 83 ec 08 e8 06 db 01 00 66 0f 1f 44 00 00 83 3d f9 2d 2c 00 00 75 10 b8 00 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 31 c3 48 83 ec 08 e8 de 9b 01 00 48 89 04 24 [ 180.394386] RSP: 002b:00007ffc14447bf8 EFLAGS: 00000246 ORIG_RAX: 0000000000000000 [ 180.398745] RAX: ffffffffffffffda RBX: 00005643f2a1a090 RCX: 00007f077c3b8910 [ 180.402710] RDX: 0000000000001000 RSI: 00005643f2a482a0 RDI: 0000000000000010 [ 180.406196] RBP: 0000000000000d68 R08: 00007f077c677178 R09: 0000000000001010 [ 180.409180] R10: 00005643f29b5410 R11: 0000000000000246 R12: 00007f077c673440 [ 180.412485] R13: 00007f077c672900 R14: 00000000ffffffff R15: 0000000000000000 [ 180.415663] ---[ end trace 9cef069cafda895a ]--- To reproduce: # build kernel cd linux cp config-5.2.0-rc2-00012-gb636fd3 .config make HOSTCC=gcc-7 CC=gcc-7 ARCH=x86_64 olddefconfig make HOSTCC=gcc-7 CC=gcc-7 ARCH=x86_64 prepare make HOSTCC=gcc-7 CC=gcc-7 ARCH=x86_64 modules_prepare make HOSTCC=gcc-7 CC=gcc-7 ARCH=x86_64 SHELL=/bin/bash make HOSTCC=gcc-7 CC=gcc-7 ARCH=x86_64 bzImage git clone https://github.com/intel/lkp-tests.git cd lkp-tests find lib/ | cpio -o -H newc --quiet | gzip > modules.cgz bin/lkp qemu -k <bzImage> -m modules.cgz job-script # job-script is attached in this email Thanks, Rong Chen