Protection of RT thread critical section

Protection of critical section

What is critical section

Critical section , In one sentence, it is a code segment that cannot be interrupted during execution . stay RT-Thread in , The most common scenario for critical segments is the operation of global variables , Global variables are like a target , Anyone can shoot him , But one shot , No one else can shoot , Otherwise, I don't know who hit the target .
So when will the critical section be interrupted ？ One is system scheduling , There is also an external interrupt . stay RT-Thread in , Finally, system scheduling is also generated PendSV interrupt , stay PendSV Handler Implementation of thread switching in , So it can be reduced to interruption . In this case ,RT-Thread The protection of critical sections is simply handled , Turn off the interrupt directly , but NMI FAULT And hard FAULT With the exception of .

Cortex-M Kernel fast shutdown interrupt instruction
In order to switch off the interrupt quickly ,Cortex-M The kernel has a special CPS Instructions , Yes 4 Usage .
Code list CPS Instruction usage

CPSID I ;PRIMASK=1　　 ; Close the interrupt 
CPSIE I ;PRIMASK=0　　 ; Open the interrupt 
CPSID F ;FAULTMASK=1　 ; Turn off the abnormality 
CPSIE F ;FAULTMASK=0　 ; Open abnormally 

In the code listing PRIMASK and FAULTMAST yes Cortex-M The kernel 3 In interrupt mask registers 2 individual , The other one is BASEPRI, About this 3 The detailed usage of registers is shown in the table .

Close the interrupt

RT-Thread The function that closes the interrupt is contex_rvds.s In the definition of , stay rthw.h In a statement , See the code list for the specific implementation 4-2.
Close the interrupt

/* ; * rt_base_t rt_hw_interrupt_disable(); ; */
rt_hw_interrupt_disable　　PROC　　　　　　　　　　
　 EXPORT　rt_hw_interrupt_disable　　　　　　　　
　 MRS　　 r0, PRIMASK　　　　　　　　　　　　　　
　 CPSID　 I　　　　　　　　　　　　　　　　　　　
　 BX　　　LR　　　　　　　　　　　　　　　　　　
　 ENDP　　　　

1）： keyword PROC Indicates that the assembly subroutine starts .
2）： Use EXPORT Keyword export label rt_hw_interrupt_disable, Make it global , After the external header file declaration （ stay rthw.h In a statement ）, You can go to C Call in file .
3）： adopt MRS The instruction will special register PRIMASK The value of the register is stored in a general-purpose register r0. When in C When the subroutine called in the assembly returns , Will r0 As the return value of the function . So in C Call in rt_hw_interrupt_disable() when , You need to declare a variable in advance to store rt_hw_interrupt_disable() The return value of , namely r0 Register value , That is to say PRIMASK Value .
4）： Turn off interrupt , That is to use CPS Instructions will be PRIMASK Set the value of the register 1.
5）： Subprogram return .
6）：ENDP Indicates the end of the assembly subroutine , And PROC Use in pairs .

Open the interrupt

RT-Thread The interrupt function is in the contex_rvds.s In the definition of , stay rthw.h In a statement .
Open the interrupt

/* * void rt_hw_interrupt_enable(rt_base_t level); */
rt_hw_interrupt_enable　　PROC　　　　　　　　　　　　
　 EXPORT　rt_hw_interrupt_enable　　　　　　　　　　
　 MSR　　 PRIMASK, r0　　　　　　　　　　　　　　　
　 BX　　　LR　　　　　　　　　　　　　　　　　　　　
　 ENDP　　　　　　　

1）： keyword PROC Indicates that the assembly subroutine starts .
2）： Use EXPORT Keyword export label rt_hw_interrupt_enable, Make it global , After the external header file declaration （ stay rthw.h In a statement ）, You can go to C Call in file .
3）： adopt MSR The instruction will the general purpose register r0 The value of is stored in a special register PRIMASK.
4）： Subprogram return .
5）：ENDP Indicates the end of the assembly subroutine , And PROC Use in pairs .

Application of critical segment code

Before entering the critical section , We will turn off the interrupt first , When exiting the critical section, open the interrupt , and Cortex-M The kernel has set the fast shutdown interrupt CPS Instructions , The implementation of switch interrupt function and the protection of critical segment code .

 The implementation of switch interrupt function and the protection of critical segment code 
 Implementation of switch interrupt function 
/* * void rt_hw_interrupt_disable(); */
rt_hw_interrupt_disable　　PROC
 EXPORT　rt_hw_interrupt_disable
　 CPSID　 I　　　　
　 BX　　　LR
　 ENDP

;/* ; * void rt_hw_interrupt_enable(void); ; */
rt_hw_interrupt_enable　　PROC
　 EXPORT　rt_hw_interrupt_enable
　 CPSIE　 I　　　　
　 BX　　　LR
　 ENDP
PRIMASK = 0;　　　　　　　　　　　/* PRIMASK The initial value is 0, It means no interruption  */　　　　

/*  Critical segment code protection  */
{
    
　 /*  The critical section begins  */
　 rt_hw_interrupt_disable();　　 /*  Close the interrupt ,PRIMASK = 1 */　　　　
　 {
    
　　　 /*  Execute critical code , Non interruptible  */　　　　
　 }
　 /*  The critical section ends  */
　　　 rt_hw_interrupt_enable();　　　/*  Open the interrupt ,PRIMASK = 0 */　　　
}