newlib: assertion failed when printf is called

[atsushi-shinbo-esoltrinity]

Describe the bug
Assertion failed when newlib/printf is called under a specific combination of configuration items.

To Reproduce

1. Modify samples/hello_world/src/main.c as following.

#include <zephyr.h>
#include <stdio.h>

void main(void)
{
	printf("Hello World! %s\n", CONFIG_BOARD);	// changed from printk() to printf()
}

2. Add following lines to samples/hello_world/prj.conf

CONFIG_ASSERT=y
CONFIG_USERSPACE=y
CONFIG_MINIMAL_LIBC=n
CONFIG_NEWLIB_LIBC=y

3. west build -b hifive1_revb samples/hello_world
4. cd build/zephyr
5. $ZEPHYR_SDK_INSTALL_DIR/riscv64-zephyr-elf/bin/riscv64-zephyr-elf-objcopy -O ihex zephyr.elf zephyr.hex
6. Write zephyr.hex to the Hifive1 revB board and run.

Conbinations	1	2	3
CONFIG_USERSPACE	y	y	n
CONFIG_MINIMAL_LIBC	n	y	n
CONFIG_NEWLIB_LIBC	y	n	y
result	NG	OK	OK

In this example, printf() is called from kernel thread.
However, when CONFIG_USERSPACE=y and CONFIG_NEWLIB_LIBC=y, the assertion has been failed.

Logs and console output

*** Booting Zephyr OS version 2.7.99  ***
ASSERTION FAIL [*lock != ((void *)0)] @ ZEPHYR_BASE/lib/libc/newlib/libc-hooks.c:378
        recursive lock allocation failed

Expected behavior

*** Booting Zephyr OS version 2.7.99  ***
Hello World! hifive1_revb

Environment (please complete the following information):

• OS: Ubuntu 18.04.5 LTS
• Toolchain: zephyr-sdk-0.13.1
• Version: v2.7.99
• Commit: 1238410
• Target: hifive1_revb

[stephanosio]

When userspace is enabled, you need to ensure the following:

1. Kernel dynamic object allocation is enabled.
2. The executing thread is assigned a resource memory pool (heap).

You can ensure (1) by setting CONFIG_DYNAMIC_OBJECTS=y.
You can ensure (2) by either:

a. Defining a dedicated heap and assigning it to the executing thread (refer to the k_thread_heap_assign documentation).
b. Enabling the default system heap and assigning it to the executing thread (set CONFIG_HEAP_MEM_POOL_SIZE to a desired value and call k_thread_system_pool_assign).

For more details, refer to the relevant documentation:
https://docs.zephyrproject.org/latest/reference/usermode/kernelobjects.html#dynamic-objects
https://docs.zephyrproject.org/latest/reference/kernel/memory/heap.html
https://docs.zephyrproject.org/latest/reference/usermode/memory_domain.html?highlight=k_thread_system_pool_assign#thread-resource-pools