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Subject[syzbot] possible deadlock in btrfs_commit_transaction
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Hello,

syzbot found the following issue on:

HEAD commit: e01d50cbd6ee Merge tag 'vfio-v6.1-rc6' of https://github.c..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=122bd401880000
kernel config: https://syzkaller.appspot.com/x/.config?x=47b0b2ecc119b39f
dashboard link: https://syzkaller.appspot.com/bug?extid=52d708a0bca2efc4c9df
compiler: Debian clang version 13.0.1-++20220126092033+75e33f71c2da-1~exp1~20220126212112.63, GNU ld (GNU Binutils for Debian) 2.35.2
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=167dfbf5880000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=11558535880000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/b48e4d485e7e/disk-e01d50cb.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/dc9ba558e055/vmlinux-e01d50cb.xz
kernel image: https://storage.googleapis.com/syzbot-assets/b766d9815123/bzImage-e01d50cb.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/ac6062c53b8d/mount_0.gz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+52d708a0bca2efc4c9df@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
6.1.0-rc5-syzkaller-00008-ge01d50cbd6ee #0 Not tainted
------------------------------------------------------
syz-executor110/3653 is trying to acquire lock:
ffff88807e4a24d8 (btrfs-root-00){.+.+}-{3:3}, at: __btrfs_tree_read_lock fs/btrfs/locking.c:134 [inline]
ffff88807e4a24d8 (btrfs-root-00){.+.+}-{3:3}, at: btrfs_tree_read_lock fs/btrfs/locking.c:140 [inline]
ffff88807e4a24d8 (btrfs-root-00){.+.+}-{3:3}, at: btrfs_read_lock_root_node+0x2b4/0x400 fs/btrfs/locking.c:279

but task is already holding lock:
ffff888027234cc0 (&fs_info->reloc_mutex){+.+.}-{3:3}, at: btrfs_commit_transaction+0x13e8/0x3760 fs/btrfs/transaction.c:2316

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #4 (&fs_info->reloc_mutex){+.+.}-{3:3}:
lock_acquire+0x182/0x3c0 kernel/locking/lockdep.c:5668
__mutex_lock_common+0x1bd/0x26e0 kernel/locking/mutex.c:603
__mutex_lock kernel/locking/mutex.c:747 [inline]
mutex_lock_nested+0x17/0x20 kernel/locking/mutex.c:799
btrfs_record_root_in_trans+0x153/0x180 fs/btrfs/transaction.c:484
start_transaction+0x3af/0x1180 fs/btrfs/transaction.c:721
btrfs_create_common+0x2c6/0x420 fs/btrfs/inode.c:6633
lookup_open fs/namei.c:3413 [inline]
open_last_lookups fs/namei.c:3481 [inline]
path_openat+0x12d0/0x2df0 fs/namei.c:3710
do_filp_open+0x264/0x4f0 fs/namei.c:3740
do_sys_openat2+0x124/0x4e0 fs/open.c:1310
do_sys_open fs/open.c:1326 [inline]
__do_sys_open fs/open.c:1334 [inline]
__se_sys_open fs/open.c:1330 [inline]
__x64_sys_open+0x221/0x270 fs/open.c:1330
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

-> #3 (btrfs_trans_num_extwriters){++++}-{0:0}:
lock_acquire+0x182/0x3c0 kernel/locking/lockdep.c:5668
join_transaction+0x19f/0xe60 fs/btrfs/transaction.c:299
start_transaction+0x6fb/0x1180 fs/btrfs/transaction.c:658
btrfs_create_common+0x2c6/0x420 fs/btrfs/inode.c:6633
lookup_open fs/namei.c:3413 [inline]
open_last_lookups fs/namei.c:3481 [inline]
path_openat+0x12d0/0x2df0 fs/namei.c:3710
do_filp_open+0x264/0x4f0 fs/namei.c:3740
do_sys_openat2+0x124/0x4e0 fs/open.c:1310
do_sys_open fs/open.c:1326 [inline]
__do_sys_open fs/open.c:1334 [inline]
__se_sys_open fs/open.c:1330 [inline]
__x64_sys_open+0x221/0x270 fs/open.c:1330
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

-> #2 (btrfs_trans_num_writers){++++}-{0:0}:
reacquire_held_locks+0x386/0x650 kernel/locking/lockdep.c:5193
__lock_release kernel/locking/lockdep.c:5382 [inline]
lock_release+0x2e2/0x820 kernel/locking/lockdep.c:5688
percpu_up_read include/linux/percpu-rwsem.h:99 [inline]
__sb_end_write include/linux/fs.h:1821 [inline]
sb_end_intwrite+0x1e/0x1a0 include/linux/fs.h:1877
__btrfs_end_transaction+0x388/0x790 fs/btrfs/transaction.c:995
btrfs_dirty_inode+0x177/0x1c0 fs/btrfs/inode.c:6099
inode_update_time fs/inode.c:1871 [inline]
touch_atime+0x315/0x630 fs/inode.c:1944
file_accessed include/linux/fs.h:2521 [inline]
btrfs_file_mmap+0xbb/0x120 fs/btrfs/file.c:2333
call_mmap include/linux/fs.h:2196 [inline]
mmap_region+0xfe6/0x1e20 mm/mmap.c:2625
do_mmap+0x8d9/0xf30 mm/mmap.c:1412
vm_mmap_pgoff+0x19e/0x2b0 mm/util.c:520
ksys_mmap_pgoff+0x48c/0x6d0 mm/mmap.c:1458
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

-> #1 (&mm->mmap_lock#2){++++}-{3:3}:
lock_acquire+0x182/0x3c0 kernel/locking/lockdep.c:5668
__might_fault+0xb2/0x110 mm/memory.c:5646
_copy_to_user+0x26/0x130 lib/usercopy.c:29
copy_to_user include/linux/uaccess.h:169 [inline]
btrfs_ioctl_get_subvol_rootref+0x8cf/0xa90 fs/btrfs/ioctl.c:3203
btrfs_ioctl+0xb7c/0xc10 fs/btrfs/ioctl.c:5556
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:870 [inline]
__se_sys_ioctl+0xfb/0x170 fs/ioctl.c:856
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

-> #0 (btrfs-root-00){.+.+}-{3:3}:
check_prev_add kernel/locking/lockdep.c:3097 [inline]
check_prevs_add kernel/locking/lockdep.c:3216 [inline]
validate_chain+0x1898/0x6ae0 kernel/locking/lockdep.c:3831
__lock_acquire+0x1292/0x1f60 kernel/locking/lockdep.c:5055
lock_acquire+0x182/0x3c0 kernel/locking/lockdep.c:5668
down_read_nested+0x3c/0x50 kernel/locking/rwsem.c:1634
__btrfs_tree_read_lock fs/btrfs/locking.c:134 [inline]
btrfs_tree_read_lock fs/btrfs/locking.c:140 [inline]
btrfs_read_lock_root_node+0x2b4/0x400 fs/btrfs/locking.c:279
btrfs_search_slot_get_root fs/btrfs/ctree.c:1665 [inline]
btrfs_search_slot+0x495/0x2fe0 fs/btrfs/ctree.c:1985
btrfs_update_root+0xf0/0xc60 fs/btrfs/root-tree.c:132
commit_fs_roots+0x4d3/0x710 fs/btrfs/transaction.c:1441
btrfs_commit_transaction+0x147f/0x3760 fs/btrfs/transaction.c:2353
iterate_supers+0x137/0x1f0 fs/super.c:724
ksys_sync+0xd5/0x1c0 fs/sync.c:104
__do_sys_sync+0xa/0x10 fs/sync.c:113
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

other info that might help us debug this:

Chain exists of:
btrfs-root-00 --> btrfs_trans_num_extwriters --> &fs_info->reloc_mutex

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&fs_info->reloc_mutex);
lock(btrfs_trans_num_extwriters);
lock(&fs_info->reloc_mutex);
lock(btrfs-root-00);

*** DEADLOCK ***

5 locks held by syz-executor110/3653:
#0: ffff8880264240e0 (&type->s_umount_key#42){++++}-{3:3}, at: iterate_supers+0xb0/0x1f0 fs/super.c:722
#1: ffff8880272363f0 (btrfs_trans_completed){.+.+}-{0:0}, at: btrfs_commit_transaction+0x130f/0x3760 fs/btrfs/transaction.c:2295
#2: ffff8880272363c8 (btrfs_trans_super_committed){.+.+}-{0:0}, at: btrfs_commit_transaction+0x133e/0x3760 fs/btrfs/transaction.c:2296
#3: ffff8880272363a0 (btrfs_trans_unblocked){.+.+}-{0:0}, at: btrfs_commit_transaction+0x136d/0x3760 fs/btrfs/transaction.c:2297
#4: ffff888027234cc0 (&fs_info->reloc_mutex){+.+.}-{3:3}, at: btrfs_commit_transaction+0x13e8/0x3760 fs/btrfs/transaction.c:2316

stack backtrace:
CPU: 1 PID: 3653 Comm: syz-executor110 Not tainted 6.1.0-rc5-syzkaller-00008-ge01d50cbd6ee #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/26/2022
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0x1b1/0x28e lib/dump_stack.c:106
check_noncircular+0x2cc/0x390 kernel/locking/lockdep.c:2177
check_prev_add kernel/locking/lockdep.c:3097 [inline]
check_prevs_add kernel/locking/lockdep.c:3216 [inline]
validate_chain+0x1898/0x6ae0 kernel/locking/lockdep.c:3831
__lock_acquire+0x1292/0x1f60 kernel/locking/lockdep.c:5055
lock_acquire+0x182/0x3c0 kernel/locking/lockdep.c:5668
down_read_nested+0x3c/0x50 kernel/locking/rwsem.c:1634
__btrfs_tree_read_lock fs/btrfs/locking.c:134 [inline]
btrfs_tree_read_lock fs/btrfs/locking.c:140 [inline]
btrfs_read_lock_root_node+0x2b4/0x400 fs/btrfs/locking.c:279
btrfs_search_slot_get_root fs/btrfs/ctree.c:1665 [inline]
btrfs_search_slot+0x495/0x2fe0 fs/btrfs/ctree.c:1985
btrfs_update_root+0xf0/0xc60 fs/btrfs/root-tree.c:132
commit_fs_roots+0x4d3/0x710 fs/btrfs/transaction.c:1441
btrfs_commit_transaction+0x147f/0x3760 fs/btrfs/transaction.c:2353
iterate_supers+0x137/0x1f0 fs/super.c:724
ksys_sync+0xd5/0x1c0 fs/sync.c:104
__do_sys_sync+0xa/0x10 fs/sync.c:113
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd
RIP: 0033:0x7fec18d5dc79
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 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 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffd705bc978 EFLAGS: 00000246 ORIG_RAX: 00000000000000a2
RAX: ffffffffffffffda RBX: 00007fec18d9ea80 RCX: 00007fec18d5dc79
RDX: 00007fec18d140c3 RSI: 0000000000000003 RDI: 0000000000004c00
RBP: 00007fec18d9e938 R08: 00005555556912c0 R09: 0000000000000140
R10: 00007ffd705bc680 R11: 0000000000000246 R12: 00007fec18d9e8d0
R13: 00007fec18d9b838 R14: 431bde82d7b634db R15: 00007ffd705bc990
</TASK>


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
syzbot can test patches for this issue, for details see:
https://goo.gl/tpsmEJ#testing-patches

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