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

Posted by kbshibukumar on 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. ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; FreeRTOSSWIHandler ; Check whether an iterrupt can occur at this point. PRESERVE8

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

vPortRestoreTaskContext portRESTORE_CONTEXT

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?

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

Posted by davedoors on March 1, 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.

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