soft lockup when reading empty file in btrfs filesystem

[vewe-richard]

Problem statement and how to reproduce

In all amazon-5.10.*, system will get lockup when reading an empty file in a btrfs disk.

Below is the way to reproduce the problem

# Any amazon-5.10.* kernel 
[ec2-user@ip-172-31-24-216 btrfs]$ uname -a
Linux ip-172-31-24-216.ap-south-1.compute.internal 5.10.184-175.749.amzn2.x86_64 #1 SMP Wed Jul 12 18:40:28 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux

sudo yum install btrfs-progs

# Create a btrfs disk, or you can get a real physical one
dd if=/dev/zero of=myfile bs=1M count=100
sudo losetup /dev/loop0 myfile 
sudo mkfs.btrfs -f /dev/loop0

sudo mount /dev/loop0 mountpoint/

sudo touch mountpoint/emptyfile

source mountpoint/emptyfile

CPU loading gets to 100% quickly, and never quit from it, after a while, the whole system may get soft lockup.

In fact,

cat mountpoint/emptyfile would not cause soft lockup, cat program calls read(file, buf, 65535).
source mountpoint/emptyfile would cause soft lockup, source command calls read(file, buf, 0) .

In a word,
When we read an empty file, and pass zero as size of bytes to read, that cause soft lockup.

The reason is, in kernel, btrfs calls generic_file_buffered_read(), which does not check a case of zero bytes to read.
It should call, generic_file_read_iter(), which will check that case and return immediately.

https://github.com/amazonlinux/linux/blob/amazon-5.10.y/master/fs/btrfs/file.c#L3599

This issue is found by us, when we are trying to replace xfs rootfs with btrfs, and got randomly soft lockup issue (which is in fact caused by crond, then run-parts to source an empty file).