Fix Non-Cooperative Core Migration + Fix yieldWithCoreMigration + Improve Mutex Locking in ConditionalVariableWait

The case of a thread not being in the core queue during a non-cooperative core affinity change would break things as the thread was non-conditionally removed and inserted, this has been fixed by adding a check to see if the thread exists in the core's queue prior to migration. In addition, `yieldWithCoreMigration` was broken by the previous commit as the fallthrough was intentional and removing it cause core migration without a yield which led to breakage in certain circumstances. The mutex locking logic was also improved in `ConditionalVariableWait` to use atomics in a more effective manner with less atomic operations being performed overall.

.idea/inspectionProfiles/Project_Default.xml

@@ -171,7 +171,7 @@
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     <inspection_tool class="CheckedExceptionClass" enabled="true" level="WARNING" enabled_by_default="true" />
     <inspection_tool class="ClangTidy" enabled="true" level="WARNING" enabled_by_default="true">
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app/src/main/cpp/skyline/kernel/scheduler.cpp

@@ -120,6 +120,27 @@ namespace skyline::kernel {
         }
     }
 
+    void Scheduler::MigrateToIdealCore(const std::shared_ptr<type::KThread> &thread, CoreContext *&core, std::unique_lock<std::mutex> &lock) {
+        // We need to check if the thread was in it's resident core's queue
+        // If it was, we need to remove it from the queue
+        auto it{std::find(core->queue.begin(), core->queue.end(), thread)};
+        bool wasInserted{it != core->queue.end()};
+        if (wasInserted) {
+            it = core->queue.erase(it);
+            if (it == core->queue.begin() && it != core->queue.end())
+                (*it)->wakeCondition.notify_one();
+        }
+        lock.unlock();
+
+        thread->coreId = thread->idealCore;
+        if (wasInserted)
+            // We need to add the thread to the ideal core queue, if it was previously it's resident core's queue
+            InsertThread(thread);
+
+        core = &cores.at(thread->coreId);
+        lock = std::unique_lock(core->mutex);
+    }
+
     void Scheduler::WaitSchedule(bool loadBalance) {
         auto &thread{state.thread};
         CoreContext *core{&cores.at(thread->coreId)};
@@ -127,14 +148,7 @@ namespace skyline::kernel {
 
         auto wakeFunction{[&]() {
             if (!thread->affinityMask.test(thread->coreId)) [[unlikely]] {
-                lock.unlock();
+                MigrateToIdealCore(thread, core, lock);
-
-                RemoveThread();
-                thread->coreId = thread->idealCore;
-                InsertThread(thread);
-
-                core = &cores.at(thread->coreId);
-                lock = std::unique_lock(core->mutex);
             }
             return !core->queue.empty() && core->queue.front() == thread;
         }};
@@ -173,14 +187,7 @@ namespace skyline::kernel {
         std::unique_lock lock(core->mutex);
         if (thread->wakeCondition.wait_for(lock, timeout, [&]() {
             if (!thread->affinityMask.test(thread->coreId)) [[unlikely]] {
-                lock.unlock();
+                MigrateToIdealCore(thread, core, lock);
-
-                RemoveThread();
-                thread->coreId = thread->idealCore;
-                InsertThread(thread);
-
-                core = &cores.at(thread->coreId);
-                lock = std::unique_lock(core->mutex);
             }
             return !core->queue.empty() && core->queue.front() == thread;
         })) {
@@ -212,8 +219,7 @@ namespace skyline::kernel {
             auto &front{core.queue.front()};
             if (front != thread)
                 front->wakeCondition.notify_one(); // If we aren't at the front of the queue, only then should we wake the thread at the front up
-        } else if (!thread->forceYield) {
+        } else if (!thread->forceYield) [[unlikely]] {
-            [[unlikely]]
             throw exception("T{} called Rotate while not being in C{}'s queue", thread->id, thread->coreId);
         }
 

app/src/main/cpp/skyline/kernel/scheduler.h

@@ -57,6 +57,12 @@ namespace skyline {
             std::mutex parkedMutex; //!< Synchronizes all operations on the queue of parked threads
             std::list<std::shared_ptr<type::KThread>> parkedQueue; //!< A queue of threads which are parked and waiting on core migration
 
+            /**
+             * @brief Migrate a thread from it's resident core to it's ideal core
+             * @note This is used to handle non-cooperative core affinity mask changes where the resident core is not in it's new affinity mask
+             */
+            void MigrateToIdealCore(const std::shared_ptr<type::KThread>& thread, CoreContext*& core, std::unique_lock<std::mutex>& lock);
+
           public:
             static constexpr std::chrono::milliseconds PreemptiveTimeslice{10}; //!< The duration of time a preemptive thread can run before yielding
             inline static int YieldSignal{SIGRTMIN}; //!< The signal used to cause a yield in running threads

app/src/main/cpp/skyline/kernel/svc.cpp

@@ -300,12 +300,12 @@ namespace skyline::kernel::svc {
         } else {
             switch (in) {
                 case yieldWithCoreMigration:
-                    state.logger->Debug("svcSleepThread: Waking any appropriate parked threads");
+                    state.logger->Debug("svcSleepThread: Waking any appropriate parked threads and yielding");
                     state.scheduler->WakeParkedThread();
-                    break;
+                    [[fallthrough]];
-
                 case yieldWithoutCoreMigration:
-                    state.logger->Debug("svcSleepThread: Cooperative Yield");
+                    if (in == yieldWithoutCoreMigration)
+                        state.logger->Debug("svcSleepThread: Cooperative yield");
                     state.scheduler->Rotate();
                     state.scheduler->WaitSchedule();
                     break;
@@ -710,7 +710,7 @@ namespace skyline::kernel::svc {
         else if (result == result::InvalidCurrentMemory)
             result = Result{}; // If the mutex value isn't expected then it's still successful
         else if (result == result::InvalidHandle)
-            state.logger->Warn("svcArbitrateLock: 'ownerHandle' invalid: 0x{:X}", ownerHandle);
+            state.logger->Warn("svcArbitrateLock: 'ownerHandle' invalid: 0x{:X} (0x{:X})", ownerHandle, mutex);
 
         state.ctx->gpr.w0 = result;
     }

app/src/main/cpp/skyline/kernel/types/KProcess.cpp

@@ -264,16 +264,14 @@ namespace skyline::kernel::type {
             state.scheduler->WaitSchedule(false);
         }
 
-        while (true) {
         KHandle value{};
+        if (!__atomic_compare_exchange_n(mutex, &value, tag, false, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST))
+            while (MutexLock(mutex, value & ~HandleWaitersBit, tag) != Result{})
+                if ((value = __atomic_or_fetch(mutex, HandleWaitersBit, __ATOMIC_SEQ_CST)) == HandleWaitersBit)
                     if (__atomic_compare_exchange_n(mutex, &value, tag, false, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST))
+                        break;
+
         return {};
-            if (!(value & HandleWaitersBit))
-                if (!__atomic_compare_exchange_n(mutex, &value, value | HandleWaitersBit, false, __ATOMIC_SEQ_CST, __ATOMIC_RELAXED))
-                    continue;
-            if (MutexLock(mutex, value & ~HandleWaitersBit, tag) == Result{})
-                return {};
-        }
     }
 
     void KProcess::ConditionalVariableSignal(u32 *key, i32 amount) {
@@ -281,7 +279,6 @@ namespace skyline::kernel::type {
         auto queue{syncWaiters.equal_range(key)};
 
         auto it{queue.first};
-        if (queue.first != queue.second)
             for (i32 waiterCount{amount}; it != queue.second && (amount <= 0 || waiterCount); it = syncWaiters.erase(it), waiterCount--)
                 state.scheduler->InsertThread(it->second);
 
@@ -330,7 +327,6 @@ namespace skyline::kernel::type {
                 return result::InvalidState;
 
         i32 waiterCount{amount};
-        if (queue.first != queue.second)
         for (auto it{queue.first}; it != queue.second && (amount <= 0 || waiterCount); it = syncWaiters.erase(it), waiterCount--)
             state.scheduler->InsertThread(it->second);