Printk processed by logger overwrites each other

[Qbicz]

Describe the bug
Enabled processing of printk messages by logger: LOG_PRINTK=y, with RTT backend, two subsequent printk calls without line break. Effect: second message is printed from beginning of the line and overwrites the first printed message.

To Reproduce
Steps to reproduce the behavior:

1. LOG_PRINTK=y
2. Use RTT backend to log
3. Whole log config:

CONFIG_LOG=y
CONFIG_LOG_DEFAULT_LEVEL=2
CONFIG_LOG_BACKEND_RTT_MODE_DROP=y
CONFIG_LOG_MODE_NO_OVERFLOW=y
CONFIG_LOG_PRINTK=y
CONFIG_LOG_PRINTK_MAX_STRING_LENGTH=256
CONFIG_LOG_PROCESS_THREAD_PRIO=10
CONFIG_LOG_BUFFER_SIZE=4096
CONFIG_LOG_BACKEND_RTT_MESSAGE_SIZE=256
CONFIG_LOG_BACKEND_SHOW_COLOR=n
CONFIG_LOG_BACKEND_FORMAT_TIMESTAMP=n

CONFIG_CONSOLE=y
CONFIG_USE_SEGGER_RTT=y
CONFIG_SEGGER_RTT_BUFFER_SIZE_UP=4096
CONFIG_RTT_CONSOLE=y
CONFIG_UART_CONSOLE=n

3. printk("1: %s ", "first_printk"); printk("2: %d ", 7); printk("\n");
4. 2: 7rst_printk

Expected behavior
Printouts should come one after another.

Impact
Incorrect printing of logs, medium priority problem.

Environment (please complete the following information):

• OS: Ubuntu 16.04 LTS
• Toolchain: Zephyr SDK 0.9.5
• Commit SHA or Version used: 9c14767
• HW: nRF52840

[Qbicz]

FYI @nordic-krch @pdunaj

[pdunaj]

It is possible that this bug is not yet completely fixed. Please check the comment in the PR.