loop2: detected capacity change from 0 to 8192
FAT-fs (loop2): Volume was not properly unmounted. Some data may be corrupt. Please run fsck.
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BUG: KCSAN: data-race in fat16_ent_put / fat_mirror_bhs

write to 0xffff88810ba5b796 of 2 bytes by task 24800 on cpu 0:
 fat16_ent_put+0x28/0x60 fs/fat/fatent.c:185
 fat_alloc_clusters+0x57b/0xc50 fs/fat/fatent.c:508
 fat_add_cluster fs/fat/inode.c:108 [inline]
 __fat_get_block fs/fat/inode.c:155 [inline]
 fat_get_block+0x25e/0x5f0 fs/fat/inode.c:190
 __block_write_begin_int+0x400/0xf90 fs/buffer.c:2042
 block_write_begin fs/buffer.c:2153 [inline]
 cont_write_begin+0x5fe/0x970 fs/buffer.c:2491
 fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
 cont_expand_zero fs/buffer.c:2419 [inline]
 cont_write_begin+0x1b0/0x970 fs/buffer.c:2481
 fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
 generic_cont_expand_simple+0xb0/0x150 fs/buffer.c:2382
 fat_cont_expand+0x3e/0x170 fs/fat/file.c:226
 fat_setattr+0x2a4/0x870 fs/fat/file.c:516
 notify_change+0x872/0x8f0 fs/attr.c:556
 do_truncate fs/open.c:68 [inline]
 do_ftruncate+0x355/0x460 fs/open.c:194
 ksys_ftruncate fs/open.c:206 [inline]
 __do_sys_ftruncate fs/open.c:211 [inline]
 __se_sys_ftruncate fs/open.c:209 [inline]
 __x64_sys_ftruncate+0x65/0xc0 fs/open.c:209
 x64_sys_call+0x2beb/0x3020 arch/x86/include/generated/asm/syscalls_64.h:78
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x12c/0x3b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88810ba5b600 of 512 bytes by task 24786 on cpu 1:
 fat_mirror_bhs+0x1df/0x330 fs/fat/fatent.c:399
 fat_ent_write+0xd0/0xe0 fs/fat/fatent.c:428
 fat_chain_add+0x172/0x490 fs/fat/misc.c:136
 fat_add_cluster fs/fat/inode.c:113 [inline]
 __fat_get_block fs/fat/inode.c:155 [inline]
 fat_get_block+0x475/0x5f0 fs/fat/inode.c:190
 __block_write_begin_int+0x400/0xf90 fs/buffer.c:2042
 block_write_begin fs/buffer.c:2153 [inline]
 cont_write_begin+0x5fe/0x970 fs/buffer.c:2491
 fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
 cont_expand_zero fs/buffer.c:2419 [inline]
 cont_write_begin+0x1b0/0x970 fs/buffer.c:2481
 fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
 generic_perform_write+0x183/0x490 mm/filemap.c:4325
 __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4442
 generic_file_write_iter+0x8d/0x310 mm/filemap.c:4468
 do_iter_readv_writev+0x4fd/0x5a0 fs/read_write.c:-1
 vfs_writev+0x2e1/0x900 fs/read_write.c:1059
 do_pwritev fs/read_write.c:1155 [inline]
 __do_sys_pwritev2 fs/read_write.c:1213 [inline]
 __se_sys_pwritev2+0xfc/0x1c0 fs/read_write.c:1204
 __x64_sys_pwritev2+0x67/0x80 fs/read_write.c:1204
 x64_sys_call+0xe07/0x3020 arch/x86/include/generated/asm/syscalls_64.h:329
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x12c/0x3b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 24786 Comm: syz.2.6967 Tainted: G        W           syzkaller #0 PREEMPT(full) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
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