From e580e087ffe393664bb9f4540b63988657b7a74e Mon Sep 17 00:00:00 2001
From: Pierre Wendling <[EMAIL REDACTED]>
Date: Tue, 6 Dec 2022 17:53:01 -0500
Subject: [PATCH] N3DS: Backport semaphore fixes from #6776.
---
docs/README-n3ds.md | 1 +
src/thread/n3ds/SDL_syssem.c | 59 ++++++++++++++++++++-------------
src/thread/n3ds/SDL_systhread.c | 3 +-
test/testsem.c | 2 ++
4 files changed, 41 insertions(+), 24 deletions(-)
diff --git a/docs/README-n3ds.md b/docs/README-n3ds.md
index 66e194d0b33a..e9e7c7d01d63 100644
--- a/docs/README-n3ds.md
+++ b/docs/README-n3ds.md
@@ -25,3 +25,4 @@ cmake --install build
- SDL2main 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 1a4109be5d6a..e7cbacb8d090 100644
--- a/src/thread/n3ds/SDL_syssem.c
+++ b/src/thread/n3ds/SDL_syssem.c
@@ -27,14 +27,16 @@
#include <3ds.h>
#include "SDL_thread.h"
+#include "SDL_timer.h"
+
+int WaitOnSemaphoreFor(SDL_sem *sem, Uint32 timeout);
struct SDL_semaphore
{
LightSemaphore semaphore;
};
-SDL_sem *
-SDL_CreateSemaphore(Uint32 initial_value)
+SDL_sem *SDL_CreateSemaphore(Uint32 initial_value)
{
SDL_sem *sem;
@@ -59,9 +61,7 @@ SDL_CreateSemaphore(Uint32 initial_value)
*/
void SDL_DestroySemaphore(SDL_sem *sem)
{
- if (sem) {
- SDL_free(sem);
- }
+ SDL_free(sem);
}
int SDL_SemTryWait(SDL_sem *sem)
@@ -70,35 +70,49 @@ int SDL_SemTryWait(SDL_sem *sem)
return SDL_InvalidParamError("sem");
}
- return SDL_SemWaitTimeout(sem, 0);
+ if (LightSemaphore_TryAcquire(&sem->semaphore, 1) != 0) {
+ /* If we failed, yield to avoid starvation on busy waits */
+ svcSleepThread(1);
+ return SDL_MUTEX_TIMEDOUT;
+ }
+
+ return 0;
}
int SDL_SemWaitTimeout(SDL_sem *sem, Uint32 timeout)
{
- int retval;
-
if (sem == NULL) {
return SDL_InvalidParamError("sem");
}
if (timeout == SDL_MUTEX_MAXWAIT) {
LightSemaphore_Acquire(&sem->semaphore, 1);
- retval = 0;
- } else {
- int return_code = LightSemaphore_TryAcquire(&sem->semaphore, 1);
- if (return_code != 0) {
- 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, timeout);
+ }
+
+ return 0;
+}
+
+int WaitOnSemaphoreFor(SDL_sem *sem, Uint32 timeout)
+{
+ Uint64 stop_time = SDL_GetTicks64() + timeout;
+ Uint64 current_time = SDL_GetTicks64();
+ 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 */
+ svcSleepThread(100000LL);
+ current_time = SDL_GetTicks64();
}
- return retval;
+ /* If we failed, yield to avoid starvation on busy waits */
+ svcSleepThread(1);
+ return SDL_MUTEX_TIMEDOUT;
}
int SDL_SemWait(SDL_sem *sem)
@@ -106,8 +120,7 @@ int SDL_SemWait(SDL_sem *sem)
return SDL_SemWaitTimeout(sem, SDL_MUTEX_MAXWAIT);
}
-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 caf2df7bb541..926dd34887ae 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 2b36a3a821a3..8d8a2493c532 100644
--- a/test/testsem.c
+++ b/test/testsem.c
@@ -210,6 +210,8 @@ TestOverheadContended(SDL_bool try_wait)
}
/* Make sure threads consumed everything */
while (SDL_SemValue(sem)) {
+ /* Friendlier with cooperative threading models */
+ SDL_Delay(1);
}
}
end_ticks = SDL_GetTicks();