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BUG: KCSAN: data-race in fat16_ent_put / fat_mirror_bhs

write to 0xffff8881062961e2 of 2 bytes by task 25179 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
 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+0x14f/0x190 fs/open.c:68
 vfs_truncate+0x1e8/0x220 fs/open.c:118
 ksys_truncate+0xa2/0x140 fs/open.c:142
 __do_sys_truncate fs/open.c:154 [inline]
 __se_sys_truncate fs/open.c:152 [inline]
 __x64_sys_truncate+0x31/0x40 fs/open.c:152
 x64_sys_call+0x19fb/0x3020 arch/x86/include/generated/asm/syscalls_64.h:77
 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 0xffff888106296000 of 512 bytes by task 25187 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+0x172/0x490 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+0x475/0x5f0 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
 __kernel_write_iter+0x319/0x590 fs/read_write.c:621
 dump_emit_page fs/coredump.c:1304 [inline]
 dump_user_range+0xa73/0xd00 fs/coredump.c:1378
 elf_core_dump+0x21a2/0x2330 fs/binfmt_elf.c:2109
 coredump_write+0xabd/0xdb0 fs/coredump.c:1053
 do_coredump fs/coredump.c:1132 [inline]
 vfs_coredump+0x26dd/0x31e0 fs/coredump.c:1206
 get_signal+0xd5c/0xf20 kernel/signal.c:3022
 arch_do_signal_or_restart+0x96/0x480 arch/x86/kernel/signal.c:337
 __exit_to_user_mode_loop kernel/entry/common.c:64 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:98 [inline]
 __exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline]
 irqentry_exit_to_user_mode_prepare include/linux/irq-entry-common.h:244 [inline]
 irqentry_exit_to_user_mode include/linux/irq-entry-common.h:315 [inline]
 irqentry_exit+0x145/0x5d0 kernel/entry/common.c:162
 asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 25187 Comm: syz.4.4748 Not tainted syzkaller #0 PREEMPT(lazy) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
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