Here are the data types on STM32 microcontrollers: http://www.keil.com/support/man/docs/armcc/armcc_chr1359125009502.htm.
These microcontrollers use 32-bit ARM core processors.
Which data types have automatic atomic read and atomic write access?
I'm pretty sure all 32-bit data types do (since the processor is 32-bits), and all 64-bit data types do NOT (since it would take at least 2 processor operations to read or write a 64-bit word), but what about bool (1 byte), and uint16_t/int16_t (2 bytes)?
Context: I'm sharing variables between multiple threads (single core, but multiple threads, or "tasks" as they are called, in FreeRTOS) on the STM32 and need to know if I need to enforce atomic access by turning off interrupts, using mutexes, etc.
UPDATE:
Refering to this sample code:
volatile bool shared_bool;
volatile uint8_t shared u8;
volatile uint16_t shared_u16;
volatile uint32_t shared_u32;
volatile uint64_t shared_u64;
volatile float shared_f; // 32-bits
volatile double shared_d; // 64-bits
// Task (thread) 1
while (1)
{
// Write to the values in this thread.
// What I write to each variable will vary. Since other threads
// are reading these values, I need to ensure my *writes* are atomic, or else
// I must use a mutex to prevent another thread from reading a variable in the middle
// of this thread's writing.
shared_bool = true;
shared_u8 = 129;
shared_u16 = 10108;
shared_u32 = 130890;
shared_f = 1083.108;
shared_d = 382.10830;
}
// Task (thread) 2
while (1)
{
// Read from the values in this thread.
// What thread 1 writes into these values can change at any time, so I need to ensure
// my *reads* are atomic, or else I'll need to use a mutex to prevent the other
// thread from writing to a variable in the midst of reading
// it in this thread.
if (shared_bool == whatever)
{
// do something
}
if (shared_u8 == whatever)
{
// do something
}
if (shared_u16 == whatever)
{
// do something
}
if (shared_u32 == whatever)
{
// do something
}
if (shared_u64 == whatever)
{
// do something
}
if (shared_f == whatever)
{
// do something
}
if (shared_d == whatever)
{
// do something
}
}
In the code above, which variables can I do this for without using a mutex? My suspicion is as follows:
volatile bool: safe--no mutex requiredvolatile uint8_t: safe--no mutex requiredvolatile uint16_t: safe--no mutex requiredvolatile uint32_t: safe--no mutex requiredvolatile uint64_t: UNSAFE--YOU MUST USE A Critical section or MUTEX!volatile float: safe--no mutex requiredvolatile double: UNSAFE--YOU MUST USE A Critical section or MUTEX!
Example critical section with FreeRTOS:
- https://www.freertos.org/taskENTER_CRITICAL_taskEXIT_CRITICAL.html
// Force atomic access with these critical section atomic access guards.
taskENTER_CRITICAL();
// do the (now guaranteed to be safe) read or write here
taskEXIT_CRITICAL();
Related, but not answering my question:
- Atomic operations in ARM
- ARM: Is writing/reading from int atomic?
- (My own question and answer on atomicity in 8-bit AVR [and Arduino] microcontrollers): https://stackoverflow.com/a/39693278/4561887
- https://stm32f4-discovery.net/2015/06/how-to-properly-enabledisable-interrupts-in-arm-cortex-m/
