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BUG: KCSAN: data-race in _copy_to_iter / fat16_ent_put

write to 0xffff88813e1d528c of 2 bytes by task 4497 on cpu 1:
 fat16_ent_put+0x28/0x60 fs/fat/fatent.c:183
 fat_alloc_clusters+0x4b5/0xa60 fs/fat/fatent.c:503
 fat_add_cluster fs/fat/inode.c:108 [inline]
 __fat_get_block fs/fat/inode.c:155 [inline]
 fat_get_block+0x25c/0x5e0 fs/fat/inode.c:190
 __block_write_begin_int+0x3b0/0xf50 fs/buffer.c:2064
 __block_write_begin fs/buffer.c:2114 [inline]
 block_write_begin+0x7c/0x180 fs/buffer.c:2175
 cont_write_begin+0x4c0/0x6f0 fs/buffer.c:2534
 fat_write_begin+0x61/0xf0 fs/fat/inode.c:230
 generic_perform_write+0x1c3/0x3d0 mm/filemap.c:3929
 __generic_file_write_iter+0xe6/0x290 mm/filemap.c:4057
 generic_file_write_iter+0x7d/0x1c0 mm/filemap.c:4089
 do_iter_write+0x4ae/0x770 fs/read_write.c:860
 vfs_iter_write+0x50/0x70 fs/read_write.c:901
 iter_file_splice_write+0x456/0x7d0 fs/splice.c:795
 do_splice_from fs/splice.c:873 [inline]
 direct_splice_actor+0x84/0xa0 fs/splice.c:1039
 splice_direct_to_actor+0x2ee/0x5f0 fs/splice.c:994
 do_splice_direct+0x104/0x180 fs/splice.c:1082
 do_sendfile+0x3b8/0x950 fs/read_write.c:1254
 __do_sys_sendfile64 fs/read_write.c:1322 [inline]
 __se_sys_sendfile64 fs/read_write.c:1308 [inline]
 __x64_sys_sendfile64+0x110/0x150 fs/read_write.c:1308
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

read to 0xffff88813e1d5000 of 1024 bytes by task 3577 on cpu 0:
 instrument_copy_to_user include/linux/instrumented.h:113 [inline]
 copyout lib/iov_iter.c:169 [inline]
 _copy_to_iter+0x142/0x940 lib/iov_iter.c:536
 copy_page_to_iter+0x1a8/0x310 lib/iov_iter.c:742
 copy_folio_to_iter include/linux/uio.h:197 [inline]
 filemap_read+0x438/0x680 mm/filemap.c:2748
 blkdev_read_iter+0x2ca/0x370 block/fops.c:606
 call_read_iter include/linux/fs.h:1862 [inline]
 new_sync_read fs/read_write.c:389 [inline]
 vfs_read+0x3b7/0x580 fs/read_write.c:470
 ksys_read+0xeb/0x1a0 fs/read_write.c:613
 __do_sys_read fs/read_write.c:623 [inline]
 __se_sys_read fs/read_write.c:621 [inline]
 __x64_sys_read+0x42/0x50 fs/read_write.c:621
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 3577 Comm: udevd Not tainted 6.4.0-rc7-syzkaller-00234-g547cc9be86f4 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/27/2023
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