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BUG: KCSAN: data-race in fat12_ent_put / fat_mirror_bhs

write to 0xffff8881502505ed of 1 bytes by task 11918 on cpu 1:
 fat12_ent_put+0x74/0x170 fs/fat/fatent.c:168
 fat_alloc_clusters+0x4ce/0xa80 fs/fat/fatent.c:503
 fat_add_cluster fs/fat/inode.c:107 [inline]
 __fat_get_block fs/fat/inode.c:154 [inline]
 fat_get_block+0x258/0x5e0 fs/fat/inode.c:189
 __block_write_begin_int+0x41c/0xf70 fs/buffer.c:2149
 block_write_begin fs/buffer.c:2260 [inline]
 cont_write_begin+0x5a3/0x8e0 fs/buffer.c:2599
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:228
 cont_expand_zero fs/buffer.c:2526 [inline]
 cont_write_begin+0x19b/0x8e0 fs/buffer.c:2589
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:228
 generic_cont_expand_simple+0xb0/0x150 fs/buffer.c:2490
 fat_cont_expand+0x3e/0x170 fs/fat/file.c:223
 fat_fallocate+0x177/0x1c0 fs/fat/file.c:301
 vfs_fallocate+0x410/0x450 fs/open.c:338
 ksys_fallocate fs/open.c:362 [inline]
 __do_sys_fallocate fs/open.c:367 [inline]
 __se_sys_fallocate fs/open.c:365 [inline]
 __x64_sys_fallocate+0x7a/0xd0 fs/open.c:365
 x64_sys_call+0x2b88/0x2fb0 arch/x86/include/generated/asm/syscalls_64.h:286
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xd0/0x1a0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888150250400 of 512 bytes by task 11938 on cpu 0:
 fat_mirror_bhs+0x1df/0x320 fs/fat/fatent.c:395
 fat_alloc_clusters+0x98b/0xa80 fs/fat/fatent.c:543
 fat_alloc_new_dir+0xbb/0x530 fs/fat/dir.c:1156
 vfat_mkdir+0x9b/0x1f0 fs/fat/namei_vfat.c:856
 vfs_mkdir+0x213/0x340 fs/namei.c:4359
 do_mkdirat+0x132/0x3f0 fs/namei.c:4392
 __do_sys_mkdir fs/namei.c:4414 [inline]
 __se_sys_mkdir fs/namei.c:4412 [inline]
 __x64_sys_mkdir+0x42/0x50 fs/namei.c:4412
 x64_sys_call+0x2289/0x2fb0 arch/x86/include/generated/asm/syscalls_64.h:84
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xd0/0x1a0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 11938 Comm: syz.0.3058 Not tainted 6.15.0-rc7-syzkaller-00007-g4a95bc121ccd #0 PREEMPT(voluntary) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
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FAT-fs (loop0): error, fat_get_cluster: invalid cluster chain (i_pos 0)