==================================================================
BUG: KCSAN: data-race in fat16_ent_put / fat_mirror_bhs

write to 0xffff8881229b02ac of 2 bytes by task 14674 on cpu 0:
 fat16_ent_put+0x28/0x60 fs/fat/fatent.c:185
 fat_ent_write+0x76/0x160 fs/fat/fatent.c:422
 fat_free fs/fat/file.c:364 [inline]
 fat_truncate_blocks+0x37a/0x5b0 fs/fat/file.c:396
 fat_write_failed fs/fat/inode.c:220 [inline]
 fat_write_end+0xba/0x160 fs/fat/inode.c:248
 generic_perform_write+0x314/0x490 mm/filemap.c:4413
 __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4509
 generic_file_write_iter+0x77/0x1f0 mm/filemap.c:4535
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x57a/0x990 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 0xffff8881229b0200 of 512 bytes by task 14688 on cpu 1:
 fat_mirror_bhs+0x1da/0x320 fs/fat/fatent.c:399
 __fat_alloc_clusters+0xc5c/0xd80 fs/fat/fatent.c:592
 fat_alloc_chain+0x25/0x30 fs/fat/fatent.c:610
 fat_add_clusters fs/fat/inode.c:109 [inline]
 __fat_get_block fs/fat/inode.c:156 [inline]
 fat_get_block+0x252/0x5e0 fs/fat/inode.c:191
 __block_write_begin_int+0x421/0x1060 fs/buffer.c:2031
 block_write_begin fs/buffer.c:2145 [inline]
 cont_write_begin+0x5bf/0x920 fs/buffer.c:2507
 fat_write_begin+0x52/0xe0 fs/fat/inode.c:231
 generic_perform_write+0x186/0x490 mm/filemap.c:4392
 __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4509
 generic_file_write_iter+0x77/0x1f0 mm/filemap.c:4535
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x57a/0x990 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

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 60929 PID: 14688 Comm: syz.5.3539 Tainted: G        W           syzkaller #0 PREEMPT(lazy) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/07/2026
==================================================================