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BUG: KCSAN: data-race in copy_page_to_iter / fat16_ent_put

write to 0xffff88803ded8ce6 of 2 bytes by task 26155 on cpu 1:
 fat16_ent_put+0x24/0x50 fs/fat/fatent.c:181
 fat_alloc_clusters+0x430/0xa60 fs/fat/fatent.c:501
 fat_add_cluster fs/fat/inode.c:108 [inline]
 __fat_get_block fs/fat/inode.c:155 [inline]
 fat_get_block+0x213/0x5a0 fs/fat/inode.c:190
 __block_write_begin_int+0x4bf/0xfd0 fs/buffer.c:2012
 __block_write_begin fs/buffer.c:2062 [inline]
 block_write_begin+0x4f/0x190 fs/buffer.c:2121
 cont_write_begin+0x42c/0x610 fs/buffer.c:2470
 fat_write_begin+0x61/0xf0 fs/fat/inode.c:235
 generic_perform_write+0x196/0x390 mm/filemap.c:3318
 __generic_file_write_iter+0x154/0x330 mm/filemap.c:3447
 generic_file_write_iter+0x2f3/0x3e0 mm/filemap.c:3479
 call_write_iter include/linux/fs.h:1908 [inline]
 new_sync_write fs/read_write.c:503 [inline]
 vfs_write+0x607/0x690 fs/read_write.c:578
 ksys_write+0xce/0x180 fs/read_write.c:631
 __do_sys_write fs/read_write.c:643 [inline]
 __se_sys_write fs/read_write.c:640 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:640
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88803ded8c00 of 1024 bytes by task 26160 on cpu 0:
 instrument_copy_to_user include/linux/instrumented.h:89 [inline]
 copyout lib/iov_iter.c:142 [inline]
 copy_page_to_iter_iovec lib/iov_iter.c:212 [inline]
 copy_page_to_iter+0x346/0x8a0 lib/iov_iter.c:921
 generic_file_buffered_read+0x10a5/0x19e0 mm/filemap.c:2130
 generic_file_read_iter+0x7d/0x3e0 mm/filemap.c:2326
 blkdev_read_iter+0xb3/0xc0 fs/block_dev.c:2044
 call_read_iter include/linux/fs.h:1902 [inline]
 new_sync_read fs/read_write.c:415 [inline]
 vfs_read+0x523/0x590 fs/read_write.c:481
 ksys_read+0xce/0x180 fs/read_write.c:607
 __do_sys_read fs/read_write.c:617 [inline]
 __se_sys_read fs/read_write.c:615 [inline]
 __x64_sys_read+0x3e/0x50 fs/read_write.c:615
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 26160 Comm: systemd-udevd Not tainted 5.8.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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