Re: [syzbot] [fs?] possible deadlock in __configfs_open_file

[syzbot <syzbot+f6e8174215573a84b797@syzkaller.appspotmail.com>]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
possible deadlock in __configfs_open_file

============================================
WARNING: possible recursive locking detected
syzkaller #0 Not tainted
--------------------------------------------
syz.0.17/6458 is trying to acquire lock:
ffff88802836c678 (&p->frag_sem){.+.+}-{4:4}, at: __configfs_open_file+0xe8/0x9c0 fs/configfs/file.c:304

but task is already holding lock:
ffff88802836c678 (&p->frag_sem){.+.+}-{4:4}, at: flush_write_buffer fs/configfs/file.c:205 [inline]
ffff88802836c678 (&p->frag_sem){.+.+}-{4:4}, at: configfs_write_iter+0x218/0x4c0 fs/configfs/file.c:229

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&p->frag_sem);
  lock(&p->frag_sem);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

5 locks held by syz.0.17/6458:
 #0: ffff888078dc50b8 (&f->f_pos_lock){+.+.}-{4:4}, at: fdget_pos+0x2aa/0x380 fs/file.c:1255
 #1: ffff88801cbe4420 (sb_writers#12){.+.+}-{0:0}, at: ksys_write+0x12a/0x250 fs/read_write.c:738
 #2: ffff888032363888 (&buffer->mutex){+.+.}-{4:4}, at: configfs_write_iter+0x76/0x4c0 fs/configfs/file.c:226
 #3: ffff88802836c678 (&p->frag_sem){.+.+}-{4:4}, at: flush_write_buffer fs/configfs/file.c:205 [inline]
 #3: ffff88802836c678 (&p->frag_sem){.+.+}-{4:4}, at: configfs_write_iter+0x218/0x4c0 fs/configfs/file.c:229
 #4: ffffffff8f65ede8 (target_devices_lock){+.+.}-{4:4}, at: target_core_item_dbroot_store+0x21/0x300 drivers/target/target_core_configfs.c:114

stack backtrace:
CPU: 1 UID: 0 PID: 6458 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/24/2026
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
 print_deadlock_bug.cold+0xbd/0xca kernel/locking/lockdep.c:3041
 check_deadlock kernel/locking/lockdep.c:3093 [inline]
 validate_chain kernel/locking/lockdep.c:3895 [inline]
 __lock_acquire+0x1287/0x2690 kernel/locking/lockdep.c:5237
 lock_acquire kernel/locking/lockdep.c:5868 [inline]
 lock_acquire+0x17c/0x330 kernel/locking/lockdep.c:5825
 down_read+0x99/0x460 kernel/locking/rwsem.c:1537
 __configfs_open_file+0xe8/0x9c0 fs/configfs/file.c:304
 do_dentry_open+0x73e/0x1570 fs/open.c:962
 vfs_open+0x82/0x3f0 fs/open.c:1094
 do_open fs/namei.c:4637 [inline]
 path_openat+0x21dc/0x3120 fs/namei.c:4796
 do_filp_open+0x1f7/0x420 fs/namei.c:4823
 file_open_name+0x198/0x3b0 fs/open.c:1376
 filp_open+0x4b/0x80 fs/open.c:1396
 target_core_item_dbroot_store+0x108/0x300 drivers/target/target_core_configfs.c:134
 flush_write_buffer fs/configfs/file.c:207 [inline]
 configfs_write_iter+0x302/0x4c0 fs/configfs/file.c:229
 new_sync_write fs/read_write.c:593 [inline]
 vfs_write+0x6ac/0x1070 fs/read_write.c:686
 ksys_write+0x12a/0x250 fs/read_write.c:738
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xc9/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f3eb239a539
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f3eb31fa028 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f3eb2605fa0 RCX: 00007f3eb239a539
RDX: 0000000000000fff RSI: 0000200000000000 RDI: 0000000000000003
RBP: 00007f3eb242dee0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f3eb2606038 R14: 00007f3eb2605fa0 R15: 00007fff2b6e3ce8
 </TASK>
db_root: not a directory: /sys/kernel/config/target/dbroot


Tested on:

commit:         05f7e89a Linux 6.19
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=124dd402580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=55c0bf5c4b466402
dashboard link: https://syzkaller.appspot.com/bug?extid=f6e8174215573a84b797
compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
patch:          https://syzkaller.appspot.com/x/patch.diff?x=149f17fa580000