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BUG: KCSAN: data-race in fat16_ent_put / fat_mirror_bhs

write to 0xffff888149043674 of 2 bytes by task 22076 on cpu 1:
 fat16_ent_put+0x28/0x60 fs/fat/fatent.c:183
 fat_ent_write+0x69/0xe0 fs/fat/fatent.c:417
 fat_chain_add+0x15d/0x440 fs/fat/misc.c:136
 fat_add_cluster fs/fat/inode.c:112 [inline]
 __fat_get_block fs/fat/inode.c:154 [inline]
 fat_get_block+0x46c/0x5e0 fs/fat/inode.c:189
 __block_write_begin_int+0x400/0xf90 fs/buffer.c:2145
 block_write_begin fs/buffer.c:2256 [inline]
 cont_write_begin+0x5fc/0x970 fs/buffer.c:2594
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:229
 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:229
 generic_cont_expand_simple+0xad/0x150 fs/buffer.c:2485
 fat_cont_expand+0x3e/0x170 fs/fat/file.c:223
 fat_setattr+0x2a5/0x8a0 fs/fat/file.c:513
 notify_change+0x809/0x890 fs/attr.c:552
 do_truncate+0x13b/0x180 fs/open.c:68
 vfs_truncate+0x1e8/0x220 fs/open.c:118
 do_sys_truncate+0x91/0x130 fs/open.c:141
 __do_sys_truncate fs/open.c:153 [inline]
 __se_sys_truncate fs/open.c:151 [inline]
 __x64_sys_truncate+0x31/0x40 fs/open.c:151
 x64_sys_call+0x1a2f/0x3000 arch/x86/include/generated/asm/syscalls_64.h:77
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888149043600 of 512 bytes by task 22096 on cpu 0:
 fat_mirror_bhs+0x1df/0x320 fs/fat/fatent.c:395
 fat_alloc_clusters+0x98b/0xa80 fs/fat/fatent.c:543
 fat_add_cluster fs/fat/inode.c:107 [inline]
 __fat_get_block fs/fat/inode.c:154 [inline]
 fat_get_block+0x258/0x5e0 fs/fat/inode.c:189
 __block_write_begin_int+0x400/0xf90 fs/buffer.c:2145
 block_write_begin fs/buffer.c:2256 [inline]
 cont_write_begin+0x5fc/0x970 fs/buffer.c:2594
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:229
 generic_perform_write+0x184/0x490 mm/filemap.c:4242
 __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4359
 generic_file_write_iter+0x8d/0x2f0 mm/filemap.c:4385
 __kernel_write_iter+0x2d3/0x540 fs/read_write.c:619
 dump_emit_page fs/coredump.c:1298 [inline]
 dump_user_range+0x61e/0x8f0 fs/coredump.c:1372
 elf_core_dump+0x1de7/0x1f80 fs/binfmt_elf.c:2111
 coredump_write+0xb0f/0xe30 fs/coredump.c:1049
 vfs_coredump+0x143a/0x20d0 fs/coredump.c:1170
 get_signal+0xd84/0xf70 kernel/signal.c:3019
 arch_do_signal_or_restart+0x96/0x440 arch/x86/kernel/signal.c:337
 exit_to_user_mode_loop kernel/entry/common.c:40 [inline]
 exit_to_user_mode_prepare include/linux/irq-entry-common.h:225 [inline]
 irqentry_exit_to_user_mode+0x5b/0xa0 kernel/entry/common.c:73
 irqentry_exit+0x12/0x50 kernel/entry/common.c:191
 asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:623

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 22096 Comm: syz.4.6157 Not tainted syzkaller #0 PREEMPT(voluntary) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
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