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Subject[syzbot] possible deadlock in btrfs_search_slot
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Hello,

syzbot found the following issue on:

HEAD commit: f0c4d9fc9cc9 Linux 6.1-rc4
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=10d158fe880000
kernel config: https://syzkaller.appspot.com/x/.config?x=771b06c41e08f95b
dashboard link: https://syzkaller.appspot.com/bug?extid=c06034aecf9f5eab1ac1
compiler: Debian clang version 13.0.1-++20220126092033+75e33f71c2da-1~exp1~20220126212112.63, GNU ld (GNU Binutils for Debian) 2.35.2

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/8c210d1cda58/disk-f0c4d9fc.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/ab3065e17b8b/vmlinux-f0c4d9fc.xz
kernel image: https://storage.googleapis.com/syzbot-assets/739b786269f4/bzImage-f0c4d9fc.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+c06034aecf9f5eab1ac1@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
6.1.0-rc4-syzkaller #0 Not tainted
------------------------------------------------------
syz-executor.1/21431 is trying to acquire lock:
ffff888028341e58 (&mm->mmap_lock#2){++++}-{3:3}, at: __might_fault+0x8f/0x110 mm/memory.c:5645

but task is already holding lock:
ffff888020a77838 (btrfs-root-00){++++}-{3:3}, at: __btrfs_tree_read_lock fs/btrfs/locking.c:134 [inline]
ffff888020a77838 (btrfs-root-00){++++}-{3:3}, at: btrfs_tree_read_lock fs/btrfs/locking.c:140 [inline]
ffff888020a77838 (btrfs-root-00){++++}-{3:3}, at: btrfs_read_lock_root_node+0x2b4/0x400 fs/btrfs/locking.c:279

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #3 (btrfs-root-00){++++}-{3:3}:
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:1649 [inline]
btrfs_search_slot+0x495/0x2fe0 fs/btrfs/ctree.c:1969
btrfs_insert_empty_items fs/btrfs/ctree.c:4103 [inline]
btrfs_insert_empty_item fs/btrfs/ctree.h:3127 [inline]
btrfs_insert_item+0x199/0x3b0 fs/btrfs/ctree.c:4132
btrfs_create_tree+0x7c9/0xc30 fs/btrfs/disk-io.c:1229
btrfs_create_free_space_tree+0x116/0x1160 fs/btrfs/free-space-tree.c:1169
btrfs_start_pre_rw_mount+0x4df/0x660 fs/btrfs/disk-io.c:3254
open_ctree+0x27e6/0x2d6d fs/btrfs/disk-io.c:3790
btrfs_fill_super+0x1c6/0x2d0 fs/btrfs/super.c:1461
btrfs_mount_root+0x885/0x9a0 fs/btrfs/super.c:1829
legacy_get_tree+0xea/0x180 fs/fs_context.c:610
vfs_get_tree+0x88/0x270 fs/super.c:1531
fc_mount fs/namespace.c:1043 [inline]
vfs_kern_mount+0xc9/0x160 fs/namespace.c:1073
btrfs_mount+0x3d3/0xbb0 fs/btrfs/super.c:1889
legacy_get_tree+0xea/0x180 fs/fs_context.c:610
vfs_get_tree+0x88/0x270 fs/super.c:1531
do_new_mount+0x289/0xad0 fs/namespace.c:3040
do_mount fs/namespace.c:3383 [inline]
__do_sys_mount fs/namespace.c:3591 [inline]
__se_sys_mount+0x2d3/0x3c0 fs/namespace.c:3568
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_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_commit_super+0x9a/0xd0 fs/btrfs/disk-io.c:4494
close_ctree+0x3a7/0xbde fs/btrfs/disk-io.c:4651
generic_shutdown_super+0x130/0x310 fs/super.c:492
kill_anon_super+0x36/0x60 fs/super.c:1086
btrfs_kill_super+0x3d/0x50 fs/btrfs/super.c:2441
deactivate_locked_super+0xa7/0xf0 fs/super.c:332
cleanup_mnt+0x494/0x520 fs/namespace.c:1186
task_work_run+0x243/0x300 kernel/task_work.c:179
resume_user_mode_work include/linux/resume_user_mode.h:49 [inline]
exit_to_user_mode_loop+0x124/0x150 kernel/entry/common.c:171
exit_to_user_mode_prepare+0xb2/0x140 kernel/entry/common.c:203
__syscall_exit_to_user_mode_work kernel/entry/common.c:285 [inline]
syscall_exit_to_user_mode+0x26/0x60 kernel/entry/common.c:296
do_syscall_64+0x49/0xb0 arch/x86/entry/common.c:86
entry_SYSCALL_64_after_hwframe+0x63/0xcd

-> #1 (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+0x10c1/0x1da0 mm/mmap.c:2625
do_mmap+0x8d9/0xf60 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

-> #0 (&mm->mmap_lock#2){++++}-{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
__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

other info that might help us debug this:

Chain exists of:
&mm->mmap_lock#2 --> btrfs_trans_num_extwriters --> btrfs-root-00

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(btrfs-root-00);
lock(btrfs_trans_num_extwriters);
lock(btrfs-root-00);
lock(&mm->mmap_lock#2);

*** DEADLOCK ***

1 lock held by syz-executor.1/21431:
#0: ffff888020a77838 (btrfs-root-00){++++}-{3:3}, at: __btrfs_tree_read_lock fs/btrfs/locking.c:134 [inline]
#0: ffff888020a77838 (btrfs-root-00){++++}-{3:3}, at: btrfs_tree_read_lock fs/btrfs/locking.c:140 [inline]
#0: ffff888020a77838 (btrfs-root-00){++++}-{3:3}, at: btrfs_read_lock_root_node+0x2b4/0x400 fs/btrfs/locking.c:279

stack backtrace:
CPU: 1 PID: 21431 Comm: syz-executor.1 Not tainted 6.1.0-rc4-syzkaller #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
__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
RIP: 0033:0x7f1a4f28b5a9
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 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f1a4ffb1168 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f1a4f3ac050 RCX: 00007f1a4f28b5a9
RDX: 0000000020058480 RSI: 00000000d000943d RDI: 0000000000000005
RBP: 00007f1a4f2e67b0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ffc4fbced1f R14: 00007f1a4ffb1300 R15: 0000000000022000
</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.

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