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BUG: KCSAN: data-race in fat16_ent_put / fat_mirror_bhs

write to 0xffff88810cd69920 of 2 bytes by task 18455 on cpu 1:
 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
 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
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x5a6/0x9f0 fs/read_write.c:688
 ksys_write+0xdc/0x1a0 fs/read_write.c:740
 __do_sys_write fs/read_write.c:751 [inline]
 __se_sys_write fs/read_write.c:748 [inline]
 __x64_sys_write+0x40/0x50 fs/read_write.c:748
 x64_sys_call+0x27e1/0x3020 arch/x86/include/generated/asm/syscalls_64.h:2
 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 0xffff88810cd69800 of 512 bytes by task 18440 on cpu 0:
 fat_mirror_bhs+0x1df/0x330 fs/fat/fatent.c:399
 fat_alloc_clusters+0xb48/0xc50 fs/fat/fatent.c:548
 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

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 18440 Comm: syz.8.3841 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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