signal: fix starvation by fast signal contexts

In the past, a signal context, that was chosen for handling by
'Signal_receiver::pending_signal and always triggered again before
the next call of 'pending_signal', caused all other contexts behind
in the list to starve. This was the case because 'pending_signal'
always took the first pending context in its context list.

We avoid this problem now by handling pending signals in a round-robin
fashion instead.

Ref #2532

repos/base-hw/src/lib/base/signal_receiver.cc

@@ -99,7 +99,7 @@ Signal_context_capability Signal_receiver::manage(Signal_context * const c)
 			/* use signal context as imprint */
 			c->_cap = pd().alloc_context(_cap, (unsigned long)c);
 			c->_receiver = this;
-			_contexts.insert(&c->_receiver_le);
+			_contexts.insert(c);
 			return c->_cap;
 		}
 		catch (Out_of_ram)  { ram_upgrade = Ram_quota { 1024*sizeof(long) }; }

repos/base/include/base/signal.h

@@ -174,6 +174,26 @@ class Genode::Signal_receiver : Noncopyable
 {
 	private:
 
+		/**
+		 * A list where the head can be moved
+		 */
+		class Context_list
+		{
+			private:
+
+				Signal_context *_head { nullptr };
+
+			public:
+
+				Signal_context *head() const { return _head; }
+
+				void head(Signal_context *re);
+
+				void insert(Signal_context *re);
+
+				void remove(Signal_context const *re);
+		};
+
 		/**
 		 * Semaphore used to indicate that signal(s) are ready to be picked
 		 * up. This is needed for platforms other than 'base-hw' only.
@@ -189,8 +209,8 @@ class Genode::Signal_receiver : Noncopyable
 		/**
 		 * List of associated contexts
 		 */
-		Lock                                _contexts_lock;
-		List<List_element<Signal_context> > _contexts;
+		Lock         _contexts_lock;
+		Context_list _contexts;
 
 		/**
 		 * Helper to dissolve given context
@@ -324,7 +344,8 @@ class Genode::Signal_context
 		/**
 		 * List element in 'Signal_receiver'
 		 */
-		List_element<Signal_context> _receiver_le;
+		Signal_context mutable *_next { nullptr };
+		Signal_context mutable *_prev { nullptr };
 
 		/**
 		 * List element in process-global registry
@@ -373,8 +394,8 @@ class Genode::Signal_context
 		 * Constructor
 		 */
 		Signal_context()
-		: _receiver_le(this), _registry_le(this), _deferred_le(this),
-		  _receiver(0), _pending(0), _ref_cnt(0) { }
+		: _registry_le(this), _deferred_le(this), _receiver(0), _pending(0),
+		  _ref_cnt(0) { }
 
 		/**
 		 * Destructor

repos/base/src/lib/base/signal.cc

@@ -224,7 +224,7 @@ Signal_context_capability Signal_receiver::manage(Signal_context *context)
 	Lock::Guard list_lock_guard(_contexts_lock);
 
 	/* insert context into context list */
-	_contexts.insert(&context->_receiver_le);
+	_contexts.insert(context);
 
 	/* register context at process-wide registry */
 	signal_context_registry()->insert(&context->_registry_le);

repos/base/src/lib/base/signal_common.cc

@@ -163,9 +163,7 @@ Signal Signal_receiver::pending_signal()
 	Lock::Guard list_lock_guard(_contexts_lock);
 
 	/* look up the contexts for the pending signal */
-	for (List_element<Signal_context> *le = _contexts.first(); le; le = le->next()) {
-
-		Signal_context *context = le->object();
+	for (Signal_context *context = _contexts.head(); context; context = context->_next) {
 
 		Lock::Guard lock_guard(context->_lock);
 
@@ -173,6 +171,7 @@ Signal Signal_receiver::pending_signal()
 		if (!context->_pending)
 			continue;
 
+		_contexts.head(context);
 		context->_pending = false;
 		Signal::Data result = context->_curr_signal;
 
@@ -206,8 +205,8 @@ Signal_receiver::~Signal_receiver()
 	Lock::Guard list_lock_guard(_contexts_lock);
 
 	/* disassociate contexts from the receiver */
-	for (List_element<Signal_context> *le; (le = _contexts.first()); )
-		_unsynchronized_dissolve(le->object());
+	while (_contexts.head())
+		_unsynchronized_dissolve(_contexts.head());
 
 	_platform_destructor();
 }
@@ -225,7 +224,7 @@ void Signal_receiver::_unsynchronized_dissolve(Signal_context * const context)
 	context->_cap      = Signal_context_capability();
 
 	/* remove context from context list */
-	_contexts.remove(&context->_receiver_le);
+	_contexts.remove(context);
 
 	_platform_finish_dissolve(context);
 }
@@ -250,9 +249,7 @@ bool Signal_receiver::pending()
 	Lock::Guard list_lock_guard(_contexts_lock);
 
 	/* look up the contexts for the pending signal */
-	for (List_element<Signal_context> *le = _contexts.first(); le; le = le->next()) {
-
-		Signal_context *context = le->object();
+	for (Signal_context *context = _contexts.head(); context; context = context->_next) {
 
 		Lock::Guard lock_guard(context->_lock);
 
@@ -261,3 +258,44 @@ bool Signal_receiver::pending()
 	}
 	return false;
 }
+
+
+void Signal_receiver::Context_list::insert(Signal_context *re)
+{
+	if (_head) {
+		re->_prev           = _head->_prev;
+		re->_next           = nullptr;
+		_head->_prev->_next = re;
+		_head->_prev        = re;
+	} else {
+		re->_prev = re;
+		re->_next = nullptr;
+		_head     = re;
+	}
+}
+
+
+void Signal_receiver::Context_list::remove(Signal_context const *re)
+{
+	if (re->_prev == re) {
+		_head = nullptr;
+	} else {
+		if (_head == re) {
+			_head =  re->_next;
+		}
+		re->_prev->_next = re->_next;
+		if (re->_next) {
+			re->_next->_prev = re->_prev;
+		} else {
+			_head->_prev = re->_prev;
+		}
+	}
+}
+
+
+void Signal_receiver::Context_list::head(Signal_context *re)
+{
+	_head->_prev->_next = _head;
+	_head = re;
+	_head->_prev->_next = nullptr;
+}