From 053ce39d67d13658f035e6ddfcae5dd1175021df Mon Sep 17 00:00:00 2001
From: Pierre Wendling <[EMAIL REDACTED]>
Date: Sun, 11 Dec 2022 14:27:16 -0500
Subject: [PATCH] N3DS: Semaphore fixes. (#6776)
* N3DS: Make Sem waits cooperative friendly.
The 3DS has a cooperative threading model. Sleeping after TryWait and
WaitTimeout avoid starving other threads. It inccurs a runtime penalty,
but it's better than having to hard reset your console to recover from
a deadlock.
---
docs/README-n3ds.md | 1 +
src/thread/n3ds/SDL_syssem.c | 53 +++++++++++++++++----------------
src/thread/n3ds/SDL_systhread.c | 3 +-
test/testsem.c | 2 ++
4 files changed, 33 insertions(+), 26 deletions(-)
diff --git a/docs/README-n3ds.md b/docs/README-n3ds.md
index 647720bc8527..933952bb1120 100644
--- a/docs/README-n3ds.md
+++ b/docs/README-n3ds.md
@@ -25,3 +25,4 @@ cmake --install build
- SDL3_main should be used to ensure ROMFS is enabled.
- By default, the extra L2 cache and higher clock speeds of the New 2/3DS lineup are enabled. If you wish to turn it off, use `osSetSpeedupEnable(false)` in your main function.
- `SDL_GetBasePath` returns the romfs root instead of the executable's directory.
+- The Nintendo 3DS uses a cooperative threading model on a single core, meaning a thread will never yield unless done manually through the `SDL_Delay` functions, or blocking waits (`SDL_LockMutex`, `SDL_SemWait`, `SDL_CondWait`, `SDL_WaitThread`). To avoid starving other threads, `SDL_SemTryWait` and `SDL_SemWaitTimeout` will yield if they fail to acquire the semaphore, see https://github.com/libsdl-org/SDL/pull/6776 for more information.
diff --git a/src/thread/n3ds/SDL_syssem.c b/src/thread/n3ds/SDL_syssem.c
index 8cd381f5c0bf..98883b0ee846 100644
--- a/src/thread/n3ds/SDL_syssem.c
+++ b/src/thread/n3ds/SDL_syssem.c
@@ -26,13 +26,14 @@
#include <3ds.h>
+int WaitOnSemaphoreFor(SDL_sem *sem, Sint64 timeout);
+
struct SDL_semaphore
{
LightSemaphore semaphore;
};
-SDL_sem *
-SDL_CreateSemaphore(Uint32 initial_value)
+SDL_sem *SDL_CreateSemaphore(Uint32 initial_value)
{
SDL_sem *sem;
@@ -57,44 +58,46 @@ SDL_CreateSemaphore(Uint32 initial_value)
*/
void SDL_DestroySemaphore(SDL_sem *sem)
{
- if (sem) {
- SDL_free(sem);
- }
+ SDL_free(sem);
}
int SDL_SemWaitTimeoutNS(SDL_sem *sem, Sint64 timeoutNS)
{
- int retval;
-
if (sem == NULL) {
return SDL_InvalidParamError("sem");
}
if (timeoutNS == SDL_MUTEX_MAXWAIT) {
LightSemaphore_Acquire(&sem->semaphore, 1);
- retval = 0;
- } else {
- int return_code = LightSemaphore_TryAcquire(&sem->semaphore, 1);
- if (return_code != 0) {
- /* FIXME: Does this code guarantee a wall clock timeout here?
- * Can we handle sub-millisecond delays? */
- u32 timeout = (u32)SDL_NS_TO_MS(timeoutNS);
- for (u32 i = 0; i < timeout; i++) {
- svcSleepThread(1000000LL);
- return_code = LightSemaphore_TryAcquire(&sem->semaphore, 1);
- if (return_code == 0) {
- break;
- }
- }
+ return 0;
+ }
+
+ if (LightSemaphore_TryAcquire(&sem->semaphore, 1) != 0) {
+ return WaitOnSemaphoreFor(sem, timeoutNS);
+ }
+
+ return 0;
+}
+
+int WaitOnSemaphoreFor(SDL_sem *sem, Sint64 timeout)
+{
+ Uint64 stop_time = SDL_GetTicksNS() + timeout;
+ Uint64 current_time = SDL_GetTicksNS();
+ while (current_time < stop_time) {
+ if (LightSemaphore_TryAcquire(&sem->semaphore, 1) == 0) {
+ return 0;
}
- retval = return_code != 0 ? SDL_MUTEX_TIMEDOUT : 0;
+ /* 100 microseconds seems to be the sweet spot */
+ SDL_DelayNS(SDL_US_TO_NS(100));
+ current_time = SDL_GetTicksNS();
}
- return retval;
+ /* If we failed, yield to avoid starvation on busy waits */
+ SDL_DelayNS(1);
+ return SDL_MUTEX_TIMEDOUT;
}
-Uint32
-SDL_SemValue(SDL_sem *sem)
+Uint32 SDL_SemValue(SDL_sem *sem)
{
if (sem == NULL) {
SDL_InvalidParamError("sem");
diff --git a/src/thread/n3ds/SDL_systhread.c b/src/thread/n3ds/SDL_systhread.c
index 7631e38f1d01..077380601092 100644
--- a/src/thread/n3ds/SDL_systhread.c
+++ b/src/thread/n3ds/SDL_systhread.c
@@ -49,8 +49,9 @@ static void ThreadEntry(void *arg)
int SDL_SYS_CreateThread(SDL_Thread *thread)
{
- s32 priority = N3DS_THREAD_PRIORITY_MEDIUM;
+ s32 priority;
size_t stack_size = GetStackSize(thread->stacksize);
+ svcGetThreadPriority(&priority, CUR_THREAD_HANDLE);
thread->handle = threadCreate(ThreadEntry,
thread,
diff --git a/test/testsem.c b/test/testsem.c
index e3a8ba217a64..9b977ba1e5c3 100644
--- a/test/testsem.c
+++ b/test/testsem.c
@@ -208,6 +208,8 @@ TestOverheadContended(SDL_bool try_wait)
}
/* Make sure threads consumed everything */
while (SDL_SemValue(sem)) {
+ /* Friendlier with cooperative threading models */
+ SDL_DelayNS(1);
}
}
end_ticks = SDL_GetTicks();