Why FreeRTOS is not masking interrupts while doing a context switch in SWI_Handler?

kbshibukumar wrote on Friday, February 28, 2014:

In the file FreeRTOS_Demo/Source/FreeRTOS-Source/Portable/RVDS/ARM-CA9/portmacro.h, we have the following definition.

#define portYIELD() __asm( “SWI 0” );

Now, the corresponding SWI handler in portASM.s is

; SVC handler is used to start the scheduler and yield a task.
; Check whether an iterrupt can occur at this point.

; Save the context of the current task and select a new task to run.
LDR R0, =vTaskSwitchContext


However, nowhere in the above code, interrupts are disabled. Doesn’t it cause any problem if any other interrupt comes and moves a task into the ready queue or schedules a new task in the mean time?

davedoors wrote on Saturday, March 01, 2014:

All IRQs are disabled by the hardware, which is why you can’t find software doing it. The IRQ handler reenables interrupts so interrupts can nest but the SVC handler doesn’t.