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BUG: KCSAN: data-race in fat16_ent_put / fat_mirror_bhs

write to 0xffff88813881134a of 2 bytes by task 10857 on cpu 0:
 fat16_ent_put+0x28/0x60 fs/fat/fatent.c:185
 fat_ent_write+0x6c/0xe0 fs/fat/fatent.c:422
 fat_chain_add+0x174/0x4a0 fs/fat/misc.c:136
 fat_add_cluster+0x77/0xe0 fs/fat/inode.c:113
 fat_fallocate+0x11e/0x1c0 fs/fat/file.c:296
 vfs_fallocate+0x3ac/0x400 fs/open.c:338
 ioctl_preallocate fs/ioctl.c:289 [inline]
 file_ioctl+0x4e1/0x5b0 fs/ioctl.c:-1
 do_vfs_ioctl+0x7c4/0xe50 fs/ioctl.c:576
 __do_sys_ioctl fs/ioctl.c:595 [inline]
 __se_sys_ioctl+0x82/0x140 fs/ioctl.c:583
 __x64_sys_ioctl+0x43/0x50 fs/ioctl.c:583
 x64_sys_call+0x1563/0x3020 arch/x86/include/generated/asm/syscalls_64.h:17
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888138811200 of 512 bytes by task 10865 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+0x174/0x4a0 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+0x468/0x5e0 fs/fat/inode.c:190
 __block_write_begin_int+0x400/0xf90 fs/buffer.c:2123
 block_write_begin fs/buffer.c:2234 [inline]
 cont_write_begin+0x5bf/0x920 fs/buffer.c:2596
 fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
 generic_perform_write+0x186/0x490 mm/filemap.c:4358
 __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4475
 generic_file_write_iter+0x77/0x1f0 mm/filemap.c:4501
 do_iter_readv_writev+0x4f6/0x5a0 fs/read_write.c:-1
 vfs_writev+0x2d1/0x8b0 fs/read_write.c:1058
 do_pwritev fs/read_write.c:1154 [inline]
 __do_sys_pwritev2 fs/read_write.c:1212 [inline]
 __se_sys_pwritev2+0xfc/0x1c0 fs/read_write.c:1203
 __x64_sys_pwritev2+0x67/0x80 fs/read_write.c:1203
 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+0x136/0x3c0 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: 10865 Comm: syz.9.1835 Tainted: G        W           syzkaller #0 PREEMPT(lazy) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
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