[syzbot] [overlayfs?] possible deadlock in mnt_want_write (9)

Hello,

syzbot found the following issue on:

HEAD commit:    cc2b5f627e8c Add linux-next specific files for 20260714
git tree:       linux-next
console+strace: https://syzkaller.appspot.com/x/log.txt?x=13631e32580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=2290ccbf984c524f
dashboard link: https://syzkaller.appspot.com/bug?extid=da381387152f0dd8bedf
compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11178db9580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16b2f4c6580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/6836f8efb1da/disk-cc2b5f62.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/0109d3477cc7/vmlinux-cc2b5f62.xz
kernel image: https://storage.googleapis.com/syzbot-assets/c2e49e350bbf/bzImage-cc2b5f62.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: [email protected]

Mass Storage Function, version: 2009/09/11
LUN: removable file: (no medium)
============================================
WARNING: possible recursive locking detected
syzkaller #0 Not tainted
--------------------------------------------
syz-executor262/5614 is trying to acquire lock:
ffff88801dfd2510 (sb_writers#8){.+.+}-{0:0}, at: mnt_want_write+0x41/0x90 fs/namespace.c:494

but task is already holding lock:
ffff88801dfd2510 (sb_writers#8){.+.+}-{0:0}, at: file_start_write include/linux/fs.h:2731 [inline]
ffff88801dfd2510 (sb_writers#8){.+.+}-{0:0}, at: vfs_write+0x231/0xbb0 fs/read_write.c:683

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

       CPU0
       ----
  lock(sb_writers#8);
  lock(sb_writers#8);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

locks held by syz-executor262/5614: 4, last CPU#1:
 #0: ffff88801dfd2510 (sb_writers#8){.+.+}-{0:0}, at: file_start_write include/linux/fs.h:2731 [inline]
 #0: ffff88801dfd2510 (sb_writers#8){.+.+}-{0:0}, at: vfs_write+0x231/0xbb0 fs/read_write.c:683
 #1: ffff888028861a78 (&buffer->mutex){+.+.}-{4:4}, at: configfs_write_iter+0x81/0x430 fs/configfs/file.c:226
 #2: ffff888033abd068 (&p->frag_sem){.+.+}-{4:4}, at: flush_write_buffer fs/configfs/file.c:205 [inline]
 #2: ffff888033abd068 (&p->frag_sem){.+.+}-{4:4}, at: configfs_write_iter+0x23b/0x430 fs/configfs/file.c:229
 #3: ffff888038383998 (&common->filesem){+.+.}-{4:4}, at: fsg_store_file+0x163/0x400 drivers/usb/gadget/function/storage_common.c:452

stack backtrace:
CPU: 1 UID: 0 PID: 5614 Comm: syz-executor262 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 print_deadlock_bug+0x27a/0x290 kernel/locking/lockdep.c:3057
 check_deadlock kernel/locking/lockdep.c:3109 [inline]
 validate_chain kernel/locking/lockdep.c:3911 [inline]
 __lock_acquire+0x261c/0x2e40 kernel/locking/lockdep.c:5254
 lock_acquire+0x115/0x350 kernel/locking/lockdep.c:5906
 percpu_down_read_internal include/linux/percpu-rwsem.h:53 [inline]
 percpu_down_read_freezable include/linux/percpu-rwsem.h:83 [inline]
 __sb_start_write include/linux/fs/super.h:19 [inline]
 sb_start_write+0x4d/0x1c0 include/linux/fs/super.h:125
 mnt_want_write+0x41/0x90 fs/namespace.c:494
 open_last_lookups fs/namei.c:4591 [inline]
 path_openat+0xa7b/0x3850 fs/namei.c:4854
 do_file_open+0x23e/0x4a0 fs/namei.c:4886
 file_open_name fs/open.c:1326 [inline]
 filp_open+0x16d/0x1d0 fs/open.c:1343
 fsg_lun_open+0x6c/0xe00 drivers/usb/gadget/function/storage_common.c:194
 fsg_store_file+0x19e/0x400 drivers/usb/gadget/function/storage_common.c:455
 flush_write_buffer fs/configfs/file.c:207 [inline]
 configfs_write_iter+0x33a/0x430 fs/configfs/file.c:229
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x61e/0xbb0 fs/read_write.c:687
 ksys_write+0x156/0x270 fs/read_write.c:739
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x17b/0x530 arch/x86/entry/syscall_64.c:85
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f79667de649
Code: c0 79 93 eb d5 48 8d 7c 1d 00 eb 99 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 d8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffe39c70488 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 0000200000000100 RCX: 00007f79667de649
RDX: 0000000000000002 RSI: 0000200000000f00 RDI: 0000000000000003
RBP: 69666e6f635f796d R08: 0000000000000006 R09: 0000000000000006
R10: 0000000000000006 R11: 0000000000000246 R12: 00007ffe39c705f8
R13: 0000200000000108 R14: 00007f7966871c40 R15: 0000000000000002
 </TASK>
mass_storage.usb0/lun.0: unable to open backing file: fi


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