ATOMIC_VALUE might return inconsistent values on avr8 (or other 8bit mcu) #6491
Description
threre might an issue in ATOMIC_VALUE(...) on avr8
#include <atomic.h>
atomic_int_t atomic = ATOMIC_INIT(0);
int atomic_val(void) {
return ATOMIC_VALUE(atomic);
}
int main(void)
{
while (1);
}
copiles to
$ BOARD=arduino-mega2560 make
$ avr-objdump -S bin/arduino-mega2560/atomic/main.o
Disassembly of section .text.atomic_val:
00000000 <atomic_val>:
#include <atomic.h>
atomic_int_t atomic = ATOMIC_INIT(0);
int atomic_val(void) {
return ATOMIC_VALUE(atomic);
0: 80 91 00 00 lds r24, 0x0000
4: 90 91 00 00 lds r25, 0x0000
}
8: 08 95 ret
Disassembly of section .text.startup.main:
00000000 <main>:
int main(void)
{
0: 00 c0 rjmp .+0 ; 0x2 <__zero_reg__+0x1>
you see two lds instructions in atomic_val()
Problem case: Suppose a higher priority thread interrupts just after the first lds and changes the value itself to 0xFFFF. In this case an inconsistent value would be returned (endianness depended either 0xFF or 0xFF00)
The correct behavior should be return either the old or the new value