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BUG: KCSAN: data-race in fat16_ent_put / fat_mirror_bhs

write to 0xffff88814efcf37c of 2 bytes by task 22409 on cpu 0:
 fat16_ent_put+0x28/0x60 fs/fat/fatent.c:183
 fat_ent_write+0x6c/0xe0 fs/fat/fatent.c:417
 fat_chain_add+0x16c/0x490 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+0x46c/0x5e0 fs/fat/inode.c:190
 __block_write_begin_int+0x400/0xf90 fs/buffer.c:2145
 block_write_begin fs/buffer.c:2256 [inline]
 cont_write_begin+0x5fe/0x970 fs/buffer.c:2594
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
 generic_perform_write+0x183/0x490 mm/filemap.c:4314
 __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4431
 generic_file_write_iter+0x8d/0x310 mm/filemap.c:4457
 new_sync_write fs/read_write.c:593 [inline]
 vfs_write+0x5a6/0x9f0 fs/read_write.c:686
 ksys_pwrite64 fs/read_write.c:793 [inline]
 __do_sys_pwrite64 fs/read_write.c:801 [inline]
 __se_sys_pwrite64 fs/read_write.c:798 [inline]
 __x64_sys_pwrite64+0xfd/0x150 fs/read_write.c:798
 x64_sys_call+0x9f7/0x3000 arch/x86/include/generated/asm/syscalls_64.h:19
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xc0/0x2a0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88814efcf200 of 512 bytes by task 22421 on cpu 1:
 fat_mirror_bhs+0x1df/0x320 fs/fat/fatent.c:395
 fat_ent_write+0xd0/0xe0 fs/fat/fatent.c:423
 fat_chain_add+0x16c/0x490 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+0x46c/0x5e0 fs/fat/inode.c:190
 __block_write_begin_int+0x400/0xf90 fs/buffer.c:2145
 block_write_begin fs/buffer.c:2256 [inline]
 cont_write_begin+0x5fe/0x970 fs/buffer.c:2594
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
 cont_expand_zero fs/buffer.c:2522 [inline]
 cont_write_begin+0x1b0/0x970 fs/buffer.c:2584
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
 generic_cont_expand_simple+0xb0/0x150 fs/buffer.c:2485
 fat_cont_expand+0x3e/0x170 fs/fat/file.c:223
 fat_fallocate+0x177/0x1c0 fs/fat/file.c:301
 vfs_fallocate+0x3b6/0x400 fs/open.c:339
 ksys_fallocate fs/open.c:363 [inline]
 __do_sys_fallocate fs/open.c:368 [inline]
 __se_sys_fallocate fs/open.c:366 [inline]
 __x64_sys_fallocate+0x7a/0xd0 fs/open.c:366
 x64_sys_call+0x1cd4/0x3000 arch/x86/include/generated/asm/syscalls_64.h:286
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xc0/0x2a0 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: 22421 Comm: syz.2.7039 Tainted: G        W           syzkaller #0 PREEMPT(voluntary) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
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