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BUG: KCSAN: data-race in _copy_to_iter / fat16_ent_put

write to 0xffff88812eadd4c6 of 2 bytes by task 6191 on cpu 0:
 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:2063
 __block_write_begin fs/buffer.c:2113 [inline]
 block_write_begin+0x7c/0x180 fs/buffer.c:2174
 cont_write_begin+0x4c0/0x6f0 fs/buffer.c:2533
 fat_write_begin+0x61/0xf0 fs/fat/inode.c:230
 generic_perform_write+0x1c3/0x3d0 mm/filemap.c:3946
 __generic_file_write_iter+0xe6/0x290 mm/filemap.c:4074
 generic_file_write_iter+0x7d/0x1c0 mm/filemap.c:4106
 do_iter_write+0x4b4/0x7b0 fs/read_write.c:860
 vfs_iter_write+0x56/0x70 fs/read_write.c:901
 iter_file_splice_write+0x456/0x7d0 fs/splice.c:762
 do_splice_from fs/splice.c:840 [inline]
 direct_splice_actor+0x8a/0xb0 fs/splice.c:1027
 splice_direct_to_actor+0x349/0x660 fs/splice.c:982
 do_splice_direct+0x104/0x180 fs/splice.c:1070
 do_sendfile+0x3b6/0x9a0 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 0xffff88812eadd000 of 1377 bytes by task 5672 on cpu 1:
 instrument_copy_to_user include/linux/instrumented.h:113 [inline]
 copyout lib/iov_iter.c:167 [inline]
 _copy_to_iter+0x12b/0x910 lib/iov_iter.c:316
 copy_page_to_iter+0x184/0x2c0 lib/iov_iter.c:481
 copy_folio_to_iter include/linux/uio.h:185 [inline]
 filemap_read+0x421/0x670 mm/filemap.c:2747
 blkdev_read_iter+0x2e1/0x380 block/fops.c:628
 call_read_iter include/linux/fs.h:1866 [inline]
 new_sync_read fs/read_write.c:389 [inline]
 vfs_read+0x3da/0x5c0 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: 1 PID: 5672 Comm: udevd Not tainted 6.4.0-syzkaller-01224-g1ef6663a587b #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/27/2023
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I/O error, dev loop1, sector 0 op 0x0:(READ) flags 0x80700 phys_seg 1 prio class 2