[Linux/Threading] Self-suspend with condition variable instead of spin

src/xenia/kernel/xboxkrnl/xboxkrnl_threading.cc

@@ -245,27 +245,17 @@ dword_result_t NtSuspendThread_entry(dword_t handle,
         return X_STATUS_THREAD_IS_TERMINATING;
       }
 #elif XE_PLATFORM_LINUX
-      // On Linux, we need to handle self-suspension specially to avoid deadlock
+      // Handle self-suspension specially to avoid deadlock.
       if (!thread->guest_object<X_KTHREAD>()->terminated) {
         bool is_self_suspend =
             (current_pcr->prcb_data.current_thread == thread->guest_object());
 
         if (is_self_suspend) {
-          // Self-suspension: increment the suspend count and wait until
-          // another thread resumes us (decrements count to 0)
-          auto guest_thread = thread->guest_object<X_KTHREAD>();
-          suspend_count = guest_thread->suspend_count;
-          guest_thread->suspend_count++;
+          XELOGD("Thread {:X} self-suspending", thread->handle());
+          suspend_count = thread->SelfSuspend();
           result = X_STATUS_SUCCESS;
-          XELOGD("Thread {:X} self-suspending (count: {}) - waiting for resume",
-                 thread->handle(), guest_thread->suspend_count);
-          // Wait until suspend_count reaches 0 (resumed by another thread)
-          while (guest_thread->suspend_count > 0) {
-            xe::threading::Sleep(std::chrono::microseconds(100));
-          }
           XELOGD("Thread {:X} resumed", thread->handle());
         } else {
-          // Normal suspension of another thread
           result = thread->Suspend(&suspend_count);
         }
       } else {

src/xenia/kernel/xthread.cc

@@ -711,7 +711,9 @@ uint32_t XThread::start_address() {
 
 X_STATUS XThread::Resume(uint32_t* out_suspend_count) {
   auto guest_thread = guest_object<X_KTHREAD>();
+  uint32_t unused_host_suspend_count = 0;
 
+#if XE_PLATFORM_WIN32
   uint8_t previous_suspend_count =
       reinterpret_cast<std::atomic_uint8_t*>(&guest_thread->suspend_count)
           ->fetch_sub(1);
@@ -719,21 +721,30 @@ X_STATUS XThread::Resume(uint32_t* out_suspend_count) {
     *out_suspend_count = previous_suspend_count;
   }
 
-  uint32_t unused_host_suspend_count = 0;
-#if XE_PLATFORM_WIN32
   if (thread_->Resume(&unused_host_suspend_count)) {
     return X_STATUS_SUCCESS;
   } else {
     return X_STATUS_UNSUCCESSFUL;
   }
 #elif XE_PLATFORM_LINUX
-  // On Linux, only resume if suspend count is 0
-  // Resume might fail if thread was self-suspended - this is expected
-  if (guest_thread->suspend_count == 0) {
-    if (!thread_->Resume(&unused_host_suspend_count)) {
-      XELOGD("Host thread resume skipped for thread {:X} (was self-suspended)",
-             handle());
+  // Use mutex to protect suspend_count access and coordinate with SelfSuspend.
+  bool should_resume_host = false;
+  {
+    std::lock_guard<std::mutex> lock(suspend_mutex_);
+    uint8_t previous = guest_thread->suspend_count;
+    if (previous > 0) {
+      guest_thread->suspend_count--;
     }
+    if (out_suspend_count) {
+      *out_suspend_count = previous;
+    }
+    should_resume_host = (guest_thread->suspend_count == 0);
+    suspend_cv_.notify_all();
+  }
+
+  // Try to resume host thread if fully resumed (for non-self-suspended case).
+  if (should_resume_host) {
+    thread_->Resume(&unused_host_suspend_count);
   }
   return X_STATUS_SUCCESS;
 #else
@@ -769,6 +780,18 @@ X_STATUS XThread::Suspend(uint32_t* out_suspend_count) {
   }
 }
 
+#if XE_PLATFORM_LINUX
+uint32_t XThread::SelfSuspend() {
+  auto guest_thread = guest_object<X_KTHREAD>();
+  std::unique_lock<std::mutex> lock(suspend_mutex_);
+  uint32_t previous = guest_thread->suspend_count;
+  guest_thread->suspend_count++;
+  suspend_cv_.wait(
+      lock, [guest_thread]() { return guest_thread->suspend_count == 0; });
+  return previous;
+}
+#endif
+
 X_STATUS XThread::Delay(uint32_t processor_mode, uint32_t alertable,
                         uint64_t interval) {
   int64_t timeout_ticks = interval;

src/xenia/kernel/xthread.h

@@ -15,6 +15,10 @@
 #include <string>
 
 #include "xenia/base/mutex.h"
+#if XE_PLATFORM_LINUX
+#include <condition_variable>
+#include <mutex>
+#endif
 #include "xenia/base/threading.h"
 #include "xenia/cpu/thread.h"
 #include "xenia/cpu/thread_state.h"
@@ -426,6 +430,13 @@ class XThread : public XObject, public cpu::Thread {
   X_STATUS Delay(uint32_t processor_mode, uint32_t alertable,
                  uint64_t interval);
 
+#if XE_PLATFORM_LINUX
+  // Performs self-suspension: increments suspend_count and blocks until
+  // another thread calls Resume() and suspend_count reaches 0.
+  // Returns the previous suspend_count value.
+  uint32_t SelfSuspend();
+#endif
+
   xe::threading::Thread* thread() { return thread_.get(); }
 
   virtual bool Save(ByteStream* stream) override;
@@ -467,6 +478,12 @@ class XThread : public XObject, public cpu::Thread {
 
   int32_t priority_ = 0;
 
+#if XE_PLATFORM_LINUX
+  // Condition variable for thread self-suspension.
+  std::mutex suspend_mutex_;
+  std::condition_variable suspend_cv_;
+#endif
+
   // Reentry context for setjmp/longjmp based stack unwinding
   std::jmp_buf reentry_jmp_buf_;
   uint32_t reentry_address_ = 0;