Strange behavior on RP2350 with mutexes and alarms #2706
Description
Hello,
I have been having some trouble with some code that makes use of mutexes to protect a data structure shared between core 0 and core 1 on the RP2350 [but the code works fine on the RP2040]. SDK version is 2.2.0.
[Note: after filing this issue I noticed that I was building with -O2 -- the problem seems to go away with optimizations turned off.]
The minimal [silly] program below demonstrates the behavior. If executed on an RP2040 [or an RP2350 with PICO_USE_SW_SPIN_LOCKS=0] the program runs as I expect with "Core 1 frames" printing and climbing at a regular cadence of ~2-3/second. There's no obvious change in anything after the timer expires.
If it is run on RP2350 with PICO_USE_SW_SPIN_LOCKS=1 [the default], the program does not advance usefully until after the timer expires - maybe 1-3 "frames" lines will be output - then the timer expires and the program advances as normal at the expected cadence of output.
Removing the add_alarm_in_ms(5000, ... call makes things proceed normally [no stall on the rp2350].
Adding a second periodic timer on a short interval [say 50ms] also seems to make things work as expected [even with USE_SW_SPIN_LOCKS=1].
Thanks for any insight and sorry if I'm missing something obvious...
#include <stdio.h>
#include <pico/multicore.h>
#include <pico/stdlib.h>
#include <pico/sync.h>
#include <pico/time.h>
#define N 1000000
auto_init_mutex(lock);
static uint32_t core1_frames = 0;
void core1_entry() {
for (;;) {
if (!mutex_enter_timeout_ms(&lock, 500)) {
continue;
}
core1_frames++;
for (volatile int i = 0; i < N; ++i);
mutex_exit(&lock);
sleep_ms(10);
}
}
int64_t do_nothing(alarm_id_t, void*) { return 0; }
void main() {
stdio_init_all();
sleep_ms(5000); // just wait for USB
printf("Starting test\n");
multicore_launch_core1(core1_entry);
add_alarm_in_ms(5000, do_nothing, NULL, true);
for (;;) {
mutex_enter_blocking(&lock);
uint32_t current = core1_frames;
for (volatile int i = 0; i < N; ++i);
mutex_exit(&lock);
printf("Core 1 Frames: %u\n", current);
sleep_ms(10);
}
}