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BUG: KASAN: null-ptr-deref in instrument_atomic_read_write include/linux/instrumented.h:101 [inline]
BUG: KASAN: null-ptr-deref in atomic_fetch_sub_release include/asm-generic/atomic-instrumented.h:220 [inline]
BUG: KASAN: null-ptr-deref in __refcount_sub_and_test include/linux/refcount.h:272 [inline]
BUG: KASAN: null-ptr-deref in __refcount_dec_and_test include/linux/refcount.h:315 [inline]
BUG: KASAN: null-ptr-deref in refcount_dec_and_test include/linux/refcount.h:333 [inline]
BUG: KASAN: null-ptr-deref in kref_put include/linux/kref.h:64 [inline]
BUG: KASAN: null-ptr-deref in bdi_put+0x22/0xa0 mm/backing-dev.c:901
Write of size 4 at addr 0000000000000040 by task syz-executor.5/16217

CPU: 1 PID: 16217 Comm: syz-executor.5 Not tainted 5.11.0-rc2-next-20210107-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:79 [inline]
 dump_stack+0x107/0x163 lib/dump_stack.c:120
 __kasan_report mm/kasan/report.c:400 [inline]
 kasan_report.cold+0x5f/0xd5 mm/kasan/report.c:413
 check_memory_region_inline mm/kasan/generic.c:180 [inline]
 check_memory_region+0x13d/0x180 mm/kasan/generic.c:186
 instrument_atomic_read_write include/linux/instrumented.h:101 [inline]
 atomic_fetch_sub_release include/asm-generic/atomic-instrumented.h:220 [inline]
 __refcount_sub_and_test include/linux/refcount.h:272 [inline]
 __refcount_dec_and_test include/linux/refcount.h:315 [inline]
 refcount_dec_and_test include/linux/refcount.h:333 [inline]
 kref_put include/linux/kref.h:64 [inline]
 bdi_put+0x22/0xa0 mm/backing-dev.c:901
 bdev_evict_inode+0x262/0x460 fs/block_dev.c:808
 evict+0x2ed/0x6b0 fs/inode.c:578
 iput_final fs/inode.c:1654 [inline]
 iput.part.0+0x57e/0x810 fs/inode.c:1680
 iput+0x58/0x70 fs/inode.c:1670
 dentry_unlink_inode+0x2b1/0x3d0 fs/dcache.c:374
 __dentry_kill+0x3c0/0x640 fs/dcache.c:579
 dentry_kill fs/dcache.c:705 [inline]
 dput+0x725/0xbc0 fs/dcache.c:885
 do_one_tree fs/dcache.c:1632 [inline]
 shrink_dcache_for_umount+0x11f/0x330 fs/dcache.c:1646
 generic_shutdown_super+0x68/0x370 fs/super.c:447
 kill_anon_super+0x36/0x60 fs/super.c:1055
 deactivate_locked_super+0x94/0x160 fs/super.c:335
 deactivate_super+0xad/0xd0 fs/super.c:366
 cleanup_mnt+0x3a3/0x530 fs/namespace.c:1110
 task_work_run+0xdd/0x190 kernel/task_work.c:140
 tracehook_notify_resume include/linux/tracehook.h:189 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:174 [inline]
 exit_to_user_mode_prepare+0x249/0x250 kernel/entry/common.c:201
 __syscall_exit_to_user_mode_work kernel/entry/common.c:291 [inline]
 syscall_exit_to_user_mode+0x19/0x50 kernel/entry/common.c:302
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x45e219
Code: 0d b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 db b3 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f90208ecc68 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffea RBX: 0000000000000005 RCX: 000000000045e219
RDX: 0000000020000180 RSI: 0000000020000100 RDI: 0000000000000000
RBP: 000000000119bfd0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 000000000119bf8c
R13: 00007fffbcd17baf R14: 00007f90208ed9c0 R15: 000000000119bf8c
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