Kernel: Don't context-switch when dispatching signals in exit_trap

We shouldn't be context-switching before the very end of this function.
The increment to m_in_critical is meant to prevent that, but
Thread::yield_without_releasing_big_lock() (potentially called by
Thread::check_dispatch_pending_signal) temporarily clears m_in_critical
and forces a context-switch regardless.

To avoid inadvertently context-switching, this patch adds a new argument
to Thread::check_dispatch_pending_signal() which can optionally make it
call Scheduler::yield() directly (which won't context switch while we're
in a critical section).

Author: implicitfield

Kernel/Arch/Processor.cpp

@@ -288,7 +288,7 @@ void ProcessorBase::exit_trap(TrapFrame& trap)
     auto* current_thread = Processor::current_thread();
     if (current_thread) {
         SpinlockLocker thread_lock(current_thread->get_lock());
-        current_thread->check_dispatch_pending_signal();
+        current_thread->check_dispatch_pending_signal(YieldBehavior::FlagYield);
     }
 
     // Process the deferred call queue. Among other things, this ensures

Kernel/Tasks/Thread.cpp

@@ -623,7 +623,7 @@ bool Thread::tick()
     return m_ticks_left != 0;
 }
 
-void Thread::check_dispatch_pending_signal()
+void Thread::check_dispatch_pending_signal(YieldBehavior yield_behavior)
 {
     auto result = DispatchSignalResult::Continue;
     {
@@ -634,7 +634,16 @@ void Thread::check_dispatch_pending_signal()
     }
 
     if (result == DispatchSignalResult::Yield) {
-        yield_without_releasing_big_lock();
+        switch (yield_behavior) {
+        case YieldBehavior::DoYield:
+            yield_without_releasing_big_lock();
+            break;
+        case YieldBehavior::FlagYield:
+            // This is essentially the same as the above,
+            // but doesn't call Processor::clear_critical().
+            Scheduler::yield();
+            break;
+        }
     }
 }
 

Kernel/Tasks/Thread.h

@@ -48,6 +48,11 @@ enum class [[nodiscard]] DispatchSignalResult {
     Continue
 };
 
+enum class YieldBehavior {
+    DoYield,
+    FlagYield
+};
+
 struct ThreadSpecificData {
     ThreadSpecificData* self;
 };
@@ -878,7 +883,7 @@ class Thread
 
     DispatchSignalResult dispatch_one_pending_signal();
     DispatchSignalResult dispatch_signal(u8 signal);
-    void check_dispatch_pending_signal();
+    void check_dispatch_pending_signal(YieldBehavior yield_behavior = YieldBehavior::DoYield);
     [[nodiscard]] bool has_unmasked_pending_signals() const { return m_have_any_unmasked_pending_signals.load(AK::memory_order_consume); }
     [[nodiscard]] bool should_ignore_signal(u8 signal) const;
     [[nodiscard]] bool has_signal_handler(u8 signal) const;