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BUG: KCSAN: data-race in copy_page_to_iter / fat16_ent_put

write to 0xffff88813070b366 of 2 bytes by task 26138 on cpu 0:
 fat16_ent_put+0x24/0x50 fs/fat/fatent.c:181
 fat_alloc_clusters+0x430/0xa50 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+0x4a2/0x1060 fs/buffer.c:2031
 __block_write_begin fs/buffer.c:2081 [inline]
 block_write_begin fs/buffer.c:2141 [inline]
 cont_write_begin+0x522/0x850 fs/buffer.c:2490
 fat_write_begin+0x61/0xf0 fs/fat/inode.c:235
 pagecache_write_begin+0x54/0x70 mm/filemap.c:3479
 cont_expand_zero fs/buffer.c:2417 [inline]
 cont_write_begin+0x164/0x850 fs/buffer.c:2480
 fat_write_begin+0x61/0xf0 fs/fat/inode.c:235
 pagecache_write_begin+0x54/0x70 mm/filemap.c:3479
 generic_cont_expand_simple+0x6d/0x100 fs/buffer.c:2381
 fat_cont_expand+0x3a/0x160 fs/fat/file.c:222
 fat_fallocate+0x17e/0x1c0 fs/fat/file.c:300
 vfs_fallocate+0x463/0x660 fs/open.c:311
 ksys_fallocate fs/open.c:334 [inline]
 __do_sys_fallocate fs/open.c:342 [inline]
 __se_sys_fallocate fs/open.c:340 [inline]
 __x64_sys_fallocate+0x75/0xc0 fs/open.c:340
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88813070b000 of 1024 bytes by task 26144 on cpu 1:
 instrument_copy_to_user include/linux/instrumented.h:119 [inline]
 copyout lib/iov_iter.c:189 [inline]
 copy_page_to_iter_iovec lib/iov_iter.c:261 [inline]
 copy_page_to_iter+0x320/0x860 lib/iov_iter.c:973
 filemap_read+0xf06/0x1230 mm/filemap.c:2605
 generic_file_read_iter+0x75/0x2c0 mm/filemap.c:2701
 blkdev_read_iter+0xb9/0xf0 fs/block_dev.c:1730
 call_read_iter include/linux/fs.h:2108 [inline]
 new_sync_read fs/read_write.c:415 [inline]
 vfs_read+0x565/0x5d0 fs/read_write.c:496
 ksys_read+0xce/0x180 fs/read_write.c:634
 __do_sys_read fs/read_write.c:644 [inline]
 __se_sys_read fs/read_write.c:642 [inline]
 __x64_sys_read+0x3e/0x50 fs/read_write.c:642
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 26144 Comm: systemd-udevd Not tainted 5.13.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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