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BUG: KCSAN: data-race in copyout / fat16_ent_put

write to 0xffff88811867331a of 2 bytes by task 25836 on cpu 1:
 fat16_ent_put+0x54/0x90 fs/fat/fatent.c:181
 fat_ent_write+0x69/0xf0 fs/fat/fatent.c:415
 fat_chain_add+0x356/0x410 fs/fat/misc.c:130
 fat_add_cluster+0x91/0xd0 fs/fat/inode.c:113
 fat_fallocate+0x13b/0x1d0 fs/fat/file.c:291
 vfs_fallocate+0x3b1/0x650 fs/open.c:309
 ioctl_preallocate+0x12a/0x190 fs/ioctl.c:504
 file_ioctl fs/ioctl.c:544 [inline]
 do_vfs_ioctl+0xa6b/0xd00 fs/ioctl.c:742
 ksys_ioctl+0xb2/0x150 fs/ioctl.c:761
 __do_sys_ioctl fs/ioctl.c:772 [inline]
 __se_sys_ioctl fs/ioctl.c:770 [inline]
 __x64_sys_ioctl+0x47/0x60 fs/ioctl.c:770
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888118673000 of 1024 bytes by task 25840 on cpu 0:
 instrument_copy_to_user include/linux/instrumented.h:89 [inline]
 copyout+0x9b/0xb0 lib/iov_iter.c:142
 copy_page_to_iter_iovec lib/iov_iter.c:212 [inline]
 copy_page_to_iter+0x25c/0x910 lib/iov_iter.c:921
 generic_file_buffered_read mm/filemap.c:2122 [inline]
 generic_file_read_iter+0x96a/0x1490 mm/filemap.c:2302
 blkdev_read_iter+0xae/0xe0 fs/block_dev.c:2039
 call_read_iter include/linux/fs.h:1895 [inline]
 new_sync_read+0x306/0x450 fs/read_write.c:414
 __vfs_read+0x9e/0xb0 fs/read_write.c:427
 vfs_read fs/read_write.c:461 [inline]
 vfs_read+0x13e/0x2b0 fs/read_write.c:446
 ksys_read+0xc5/0x1a0 fs/read_write.c:587
 __do_sys_read fs/read_write.c:597 [inline]
 __se_sys_read fs/read_write.c:595 [inline]
 __x64_sys_read+0x49/0x60 fs/read_write.c:595
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 25840 Comm: blkid Not tainted 5.6.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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