syzbot

loop5: detected capacity change from 0 to 8192
======================================================
WARNING: possible circular locking dependency detected
6.12.0-syzkaller-11930-g0e287d31b62b #0 Not tainted
------------------------------------------------------
syz.5.2049/20355 is trying to acquire lock:
ffff88804d8140b0 (&sbi->fat_lock){+.+.}-{4:4}, at: lock_fat fs/fat/fatent.c:281 [inline]
ffff88804d8140b0 (&sbi->fat_lock){+.+.}-{4:4}, at: fat_count_free_clusters+0x138/0x9d0 fs/fat/fatent.c:724

but task is already holding lock:
ffff88814330df00 (&q->limits_lock){+.+.}-{4:4}, at: queue_limits_start_update include/linux/blkdev.h:945 [inline]
ffff88814330df00 (&q->limits_lock){+.+.}-{4:4}, at: loop_reconfigure_limits+0x2da/0x8d0 drivers/block/loop.c:1003

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:585 [inline]
       __mutex_lock+0x19b/0xa60 kernel/locking/mutex.c:735
       queue_limits_start_update include/linux/blkdev.h:945 [inline]
       loop_reconfigure_limits+0x2da/0x8d0 drivers/block/loop.c:1003
       loop_set_block_size drivers/block/loop.c:1473 [inline]
       lo_simple_ioctl drivers/block/loop.c:1496 [inline]
       lo_ioctl+0x901/0x18b0 drivers/block/loop.c:1559
       blkdev_ioctl+0x279/0x6d0 block/ioctl.c:693
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:906 [inline]
       __se_sys_ioctl fs/ioctl.c:892 [inline]
       __x64_sys_ioctl+0x193/0x200 fs/ioctl.c:892
       do_syscall_x64 arch/x86/entry/common.c:52 [inline]
       do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (&q->q_usage_counter(io)
#24){++++}-{0:0}:
       bio_queue_enter block/blk.h:75 [inline]
       blk_mq_submit_bio+0x1fb6/0x24c0 block/blk-mq.c:3092
       __submit_bio+0x384/0x540 block/blk-core.c:629
       __submit_bio_noacct_mq block/blk-core.c:710 [inline]
       submit_bio_noacct_nocheck+0x698/0xd70 block/blk-core.c:739
       submit_bio_noacct+0x93a/0x1e20 block/blk-core.c:868
       submit_bh fs/buffer.c:2819 [inline]
       __bread_slow fs/buffer.c:1264 [inline]
       __bread_gfp+0x189/0x340 fs/buffer.c:1488
       sb_bread include/linux/buffer_head.h:346 [inline]
       fat12_ent_bread+0x141/0x410 fs/fat/fatent.c:77
       fat_ent_read+0x6a4/0xb90 fs/fat/fatent.c:368
       fat_free_clusters+0x1b7/0xa60 fs/fat/fatent.c:568
       fat_free fs/fat/file.c:376 [inline]
       fat_truncate_blocks+0x53a/0xbe0 fs/fat/file.c:394
       fat_evict_inode+0x8d/0x280 fs/fat/inode.c:656
       evict+0x40c/0x960 fs/inode.c:796
       iput_final fs/inode.c:1946 [inline]
       iput fs/inode.c:1972 [inline]
       iput+0x52a/0x890 fs/inode.c:1958
       do_unlinkat+0x5c3/0x760 fs/namei.c:4594
       __do_sys_unlinkat fs/namei.c:4630 [inline]
       __se_sys_unlinkat fs/namei.c:4623 [inline]
       __x64_sys_unlinkat+0xbf/0x130 fs/namei.c:4623
       do_syscall_x64 arch/x86/entry/common.c:52 [inline]
       do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&sbi->fat_lock){+.+.}-{4:4}:
       check_prev_add kernel/locking/lockdep.c:3161 [inline]
       check_prevs_add kernel/locking/lockdep.c:3280 [inline]
       validate_chain kernel/locking/lockdep.c:3904 [inline]
       __lock_acquire+0x249e/0x3c40 kernel/locking/lockdep.c:5226
       lock_acquire.part.0+0x11b/0x380 kernel/locking/lockdep.c:5849
       __mutex_lock_common kernel/locking/mutex.c:585 [inline]
       __mutex_lock+0x19b/0xa60 kernel/locking/mutex.c:735
       lock_fat fs/fat/fatent.c:281 [inline]
       fat_count_free_clusters+0x138/0x9d0 fs/fat/fatent.c:724
       fat_statfs+0x3c2/0x4d0 fs/fat/inode.c:834
       statfs_by_dentry+0x12e/0x210 fs/statfs.c:66
       vfs_statfs+0x3a/0xa0 fs/statfs.c:90
       loop_config_discard drivers/block/loop.c:798 [inline]
       loop_reconfigure_limits+0x5b7/0x8d0 drivers/block/loop.c:1012
       loop_configure+0x7a0/0x11f0 drivers/block/loop.c:1093
       lo_ioctl+0xa6a/0x18b0 drivers/block/loop.c:1532
       blkdev_ioctl+0x279/0x6d0 block/ioctl.c:693
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:906 [inline]
       __se_sys_ioctl fs/ioctl.c:892 [inline]
       __x64_sys_ioctl+0x193/0x200 fs/ioctl.c:892
       do_syscall_x64 arch/x86/entry/common.c:52 [inline]
       do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83
       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 ***

2 locks held by syz.5.2049/20355:
 #0: ffff888145ac7360 (&lo->lo_mutex){+.+.}-{4:4}, at: loop_global_lock_killable drivers/block/loop.c:120 [inline]
 #0: ffff888145ac7360 (&lo->lo_mutex){+.+.}-{4:4}, at: loop_configure+0x22f/0x11f0 drivers/block/loop.c:1044
 #1: ffff88814330df00 (&q->limits_lock){+.+.}-{4:4}, at: queue_limits_start_update include/linux/blkdev.h:945 [inline]
 #1: ffff88814330df00 (&q->limits_lock){+.+.}-{4:4}, at: loop_reconfigure_limits+0x2da/0x8d0 drivers/block/loop.c:1003

stack backtrace:
CPU: 1 UID: 0 PID: 20355 Comm: syz.5.2049 Not tainted 6.12.0-syzkaller-11930-g0e287d31b62b #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
 print_circular_bug+0x419/0x5d0 kernel/locking/lockdep.c:2074
 check_noncircular+0x31a/0x400 kernel/locking/lockdep.c:2206
 check_prev_add kernel/locking/lockdep.c:3161 [inline]
 check_prevs_add kernel/locking/lockdep.c:3280 [inline]
 validate_chain kernel/locking/lockdep.c:3904 [inline]
 __lock_acquire+0x249e/0x3c40 kernel/locking/lockdep.c:5226
 lock_acquire.part.0+0x11b/0x380 kernel/locking/lockdep.c:5849
 __mutex_lock_common kernel/locking/mutex.c:585 [inline]
 __mutex_lock+0x19b/0xa60 kernel/locking/mutex.c:735
 lock_fat fs/fat/fatent.c:281 [inline]
 fat_count_free_clusters+0x138/0x9d0 fs/fat/fatent.c:724
 fat_statfs+0x3c2/0x4d0 fs/fat/inode.c:834
 statfs_by_dentry+0x12e/0x210 fs/statfs.c:66
 vfs_statfs+0x3a/0xa0 fs/statfs.c:90
 loop_config_discard drivers/block/loop.c:798 [inline]
 loop_reconfigure_limits+0x5b7/0x8d0 drivers/block/loop.c:1012
 loop_configure+0x7a0/0x11f0 drivers/block/loop.c:1093
 lo_ioctl+0xa6a/0x18b0 drivers/block/loop.c:1532
 blkdev_ioctl+0x279/0x6d0 block/ioctl.c:693
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:906 [inline]
 __se_sys_ioctl fs/ioctl.c:892 [inline]
 __x64_sys_ioctl+0x193/0x200 fs/ioctl.c:892
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fe588180849
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 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fe589035058 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007fe588345fa0 RCX: 00007fe588180849
RDX: 00000000200002c0 RSI: 0000000000004c0a RDI: 0000000000000004
RBP: 00007fe5881f3986 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: 00007fe588345fa0 R15: 00007fff267de898
 </TASK>