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BUG: KCSAN: data-race in fat16_ent_put / fat_mirror_bhs

write to 0xffff88810bde2ecc of 2 bytes by task 3548 on cpu 1:
 fat16_ent_put+0x28/0x60 fs/fat/fatent.c:185
 fat_ent_write+0x6c/0xe0 fs/fat/fatent.c:422
 fat_free fs/fat/file.c:366 [inline]
 fat_truncate_blocks+0x376/0x5b0 fs/fat/file.c:397
 fat_write_failed fs/fat/inode.c:219 [inline]
 fat_write_end+0xba/0x160 fs/fat/inode.c:247
 generic_perform_write+0x314/0x490 mm/filemap.c:4395
 __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4491
 generic_file_write_iter+0x77/0x1f0 mm/filemap.c:4517
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x57f/0x9a0 fs/read_write.c:687
 ksys_write+0xdc/0x1a0 fs/read_write.c:739
 __do_sys_write fs/read_write.c:750 [inline]
 __se_sys_write fs/read_write.c:747 [inline]
 __x64_sys_write+0x40/0x50 fs/read_write.c:747
 x64_sys_call+0x154e/0x2550 arch/x86/include/generated/asm/syscalls_64.h:2
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x112/0x360 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88810bde2e00 of 512 bytes by task 3538 on cpu 0:
 fat_mirror_bhs+0x1df/0x330 fs/fat/fatent.c:399
 fat_ent_write+0xd0/0xe0 fs/fat/fatent.c:428
 fat_chain_add+0x170/0x4c0 fs/fat/misc.c:138
 fat_add_cluster+0x77/0xe0 fs/fat/inode.c:113
 fat_fallocate+0x11e/0x1c0 fs/fat/file.c:295
 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+0x239d/0x2550 arch/x86/include/generated/asm/syscalls_64.h:17
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x112/0x360 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 3538 Comm: syz.1.8 Not tainted syzkaller #0 PREEMPT(lazy) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
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