[syzbot] [exfat?] possible deadlock in fat_count_free_clusters (2)

Hello,

syzbot found the following issue on:

HEAD commit:    28d012efb432 Add linux-next specific files for 20260812
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=154f8ac6580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=5a13c7bc480b5c3f
dashboard link: https://syzkaller.appspot.com/bug?extid=e6f6bc2e3e041d4d72dd
compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/de882be6df4b/disk-28d012ef.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/412cb9fb7e2d/vmlinux-28d012ef.xz
kernel image: https://storage.googleapis.com/syzbot-assets/9d7096c27f1b/bzImage-28d012ef.xz

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

loop2: detected capacity change from 0 to 128
FAT-fs (loop2): utf8 is not a recommended IO charset for FAT filesystems, filesystem will be case sensitive!
FAT-fs (loop2): Invalid FSINFO signature: 0x41615252, 0x80417272 (sector = 1)
======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Tainted: G             L     
------------------------------------------------------
syz.2.7389/30564 is trying to acquire lock:
ffff888076f5c0a8 (&sbi->fat_lock){+.+.}-{4:4}, at: lock_fat fs/fat/fatent.c:285 [inline]
ffff888076f5c0a8 (&sbi->fat_lock){+.+.}-{4:4}, at: fat_count_free_clusters+0x157/0xb40 fs/fat/fatent.c:729

but task is already holding lock:
ffff888024a13528 (&q->limits_lock){+.+.}-{4:4}, at: queue_limits_start_update include/linux/blkdev.h:1101 [inline]
ffff888024a13528 (&q->limits_lock){+.+.}-{4:4}, at: loop_configure+0x787/0xdc0 drivers/block/loop.c:1083

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (&q->limits_lock){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/mutex.c:646 [inline]
       __mutex_lock+0x19d/0x1550 kernel/locking/mutex.c:821
       queue_limits_start_update include/linux/blkdev.h:1101 [inline]
       loop_set_dio drivers/block/loop.c:1460 [inline]
       lo_simple_ioctl drivers/block/loop.c:1528 [inline]
       lo_ioctl+0x16ca/0x2150 drivers/block/loop.c:1594
       blkdev_ioctl+0x5e3/0x740 block/ioctl.c:798
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:597 [inline]
       __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
       do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
       do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (&q->q_usage_counter(io)#24){++++}-{0:0}:
       bio_queue_enter block/blk.h:95 [inline]
       blk_mq_submit_bio+0x7f8/0x2a70 block/blk-mq.c:3127
       __submit_bio_noacct_mq block/blk-core.c:756 [inline]
       submit_bio_noacct_nocheck+0x2f4/0xa40 block/blk-core.c:790
       bio_await block/bio.c:1578 [inline]
       submit_bio_wait+0x12a/0x250 block/bio.c:1596
       blkdev_issue_discard+0x133/0x1e0 block/blk-lib.c:94
       sb_issue_discard include/linux/blkdev.h:1207 [inline]
       fat_free_clusters+0x430/0xbf0 fs/fat/fatent.c:593
       fat_free fs/fat/file.c:379 [inline]
       fat_truncate_blocks+0x896/0xc00 fs/fat/file.c:397
       fat_write_failed fs/fat/inode.c:219 [inline]
       fat_write_begin+0x127/0x150 fs/fat/inode.c:234
       cont_expand_zero fs/buffer.c:2434 [inline]
       cont_write_begin+0x2d6/0xac0 fs/buffer.c:2496
       fat_write_begin+0x7a/0x150 fs/fat/inode.c:230
       generic_perform_write+0x2d5/0x8f0 mm/filemap.c:4374
       generic_file_write_iter+0xae/0x330 mm/filemap.c:4517
       new_sync_write fs/read_write.c:595 [inline]
       vfs_write+0x612/0xba0 fs/read_write.c:687
       ksys_pwrite64 fs/read_write.c:794 [inline]
       __do_sys_pwrite64 fs/read_write.c:802 [inline]
       __se_sys_pwrite64 fs/read_write.c:799 [inline]
       __x64_sys_pwrite64+0x196/0x220 fs/read_write.c:799
       do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
       do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&sbi->fat_lock){+.+.}-{4:4}:
       check_prev_add kernel/locking/lockdep.c:3181 [inline]
       check_prevs_add kernel/locking/lockdep.c:3300 [inline]
       validate_chain kernel/locking/lockdep.c:3924 [inline]
       __lock_acquire+0x15ff/0x2e50 kernel/locking/lockdep.c:5254
       lock_acquire+0x115/0x350 kernel/locking/lockdep.c:5908
       __mutex_lock_common kernel/locking/mutex.c:646 [inline]
       __mutex_lock+0x19d/0x1550 kernel/locking/mutex.c:821
       lock_fat fs/fat/fatent.c:285 [inline]
       fat_count_free_clusters+0x157/0xb40 fs/fat/fatent.c:729
       fat_statfs+0x135/0x400 fs/fat/inode.c:874
       statfs_by_dentry fs/statfs.c:66 [inline]
       vfs_statfs+0x147/0x2d0 fs/statfs.c:90
       loop_get_discard_config drivers/block/loop.c:774 [inline]
       loop_update_limits+0x375/0x920 drivers/block/loop.c:984
       loop_configure+0x7d2/0xdc0 drivers/block/loop.c:1084
       lo_ioctl+0xad5/0x2150 drivers/block/loop.c:1564
       blkdev_ioctl+0x5e3/0x740 block/ioctl.c:798
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:597 [inline]
       __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
       do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
       do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

Chain exists of:
  &sbi->fat_lock --> &q->q_usage_counter(io)#24 --> &q->limits_lock

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&q->limits_lock);
                               lock(&q->q_usage_counter(io)#24);
                               lock(&q->limits_lock);
  lock(&sbi->fat_lock);

 *** DEADLOCK ***

locks held by syz.2.7389/30564: 2, last CPU#1:
 #0: ffff888024aba430 (&lo->lo_mutex){+.+.}-{4:4}, at: loop_global_lock_killable drivers/block/loop.c:119 [inline]
 #0: ffff888024aba430 (&lo->lo_mutex){+.+.}-{4:4}, at: loop_configure+0x35d/0xdc0 drivers/block/loop.c:1038
 #1: ffff888024a13528 (&q->limits_lock){+.+.}-{4:4}, at: queue_limits_start_update include/linux/blkdev.h:1101 [inline]
 #1: ffff888024a13528 (&q->limits_lock){+.+.}-{4:4}, at: loop_configure+0x787/0xdc0 drivers/block/loop.c:1083

stack backtrace:
CPU: 1 UID: 0 PID: 30564 Comm: syz.2.7389 Tainted: G             L      syzkaller #0 PREEMPT(full) 
Tainted: [L]=SOFTLOCKUP
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_circular_bug+0x2e2/0x300 kernel/locking/lockdep.c:2059
 check_noncircular+0x12f/0x150 kernel/locking/lockdep.c:2191
 check_prev_add kernel/locking/lockdep.c:3181 [inline]
 check_prevs_add kernel/locking/lockdep.c:3300 [inline]
 validate_chain kernel/locking/lockdep.c:3924 [inline]
 __lock_acquire+0x15ff/0x2e50 kernel/locking/lockdep.c:5254
 lock_acquire+0x115/0x350 kernel/locking/lockdep.c:5908
 __mutex_lock_common kernel/locking/mutex.c:646 [inline]
 __mutex_lock+0x19d/0x1550 kernel/locking/mutex.c:821
 lock_fat fs/fat/fatent.c:285 [inline]
 fat_count_free_clusters+0x157/0xb40 fs/fat/fatent.c:729
 fat_statfs+0x135/0x400 fs/fat/inode.c:874
 statfs_by_dentry fs/statfs.c:66 [inline]
 vfs_statfs+0x147/0x2d0 fs/statfs.c:90
 loop_get_discard_config drivers/block/loop.c:774 [inline]
 loop_update_limits+0x375/0x920 drivers/block/loop.c:984
 loop_configure+0x7d2/0xdc0 drivers/block/loop.c:1084
 lo_ioctl+0xad5/0x2150 drivers/block/loop.c:1564
 blkdev_ioctl+0x5e3/0x740 block/ioctl.c:798
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f704bd9e0d9
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:00007f704cc92028 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f704c025fa0 RCX: 00007f704bd9e0d9
RDX: 00002000000005c0 RSI: 0000000000004c0a RDI: 0000000000000004
RBP: 00007f704be35024 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f704c026038 R14: 00007f704c025fa0 R15: 00007ffda9ce6638
 </TASK>


---
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