core: equip signal-context slab with initial block

By supplying a statically allocated initial block to the slab allocator
for signal contexts, we become able to construct a 'Signal_broker' (the
back end for the PD's signalling API) without any dynamic memory
allocation. This is a precondition for using the PD as meta-data
allocator for its contained signal broker (meta data allocations must
not happen before the PD construction is complete).

Issue #2407

repos/base-nova/src/core/include/signal_broker.h

@@ -24,6 +24,7 @@
 #include <platform.h>
 #include <signal_source_component.h>
 #include <signal_source/capability.h>
+#include <signal_context_slab.h>
 
 namespace Genode { class Signal_broker; }
 
@@ -38,8 +39,7 @@ class Genode::Signal_broker
 		Rpc_entrypoint                       &_context_ep;
 		Signal_source_component               _source;
 		Signal_source_capability              _source_cap;
-		Tslab<Signal_context_component,
-		      960*sizeof(long)>               _contexts_slab { &_md_alloc };
+		Signal_context_slab                   _context_slab { _md_alloc };
 
 	public:
 
@@ -62,7 +62,7 @@ class Genode::Signal_broker
 			_source_ep.dissolve(&_source);
 
 			/* free all signal contexts */
-			while (Signal_context_component *r = _contexts_slab.first_object())
+			while (Signal_context_component *r = _context_slab.any_signal_context())
 				free_context(reinterpret_cap_cast<Signal_context>(r->cap()));
 		}
 
@@ -101,7 +101,7 @@ class Genode::Signal_broker
 			}
 
 			/* the _contexts_slab may throw Allocator::Out_of_memory */
-			_obj_pool.insert(new (&_contexts_slab) Signal_context_component(cap));
+			_obj_pool.insert(new (&_context_slab) Signal_context_component(cap));
 
 			/* return unique capability for the signal context */
 			return cap;
@@ -121,7 +121,7 @@ class Genode::Signal_broker
 				        Hex(context_cap.local_name()));
 				return;
 			}
-			destroy(&_contexts_slab, context);
+			destroy(&_context_slab, context);
 
 			Nova::revoke(Nova::Obj_crd(context_cap.local_name(), 0));
 			cap_map()->remove(context_cap.local_name(), 0);

repos/base/src/core/include/signal_broker.h

@@ -16,6 +16,7 @@
 
 #include <signal_source_component.h>
 #include <signal_source/capability.h>
+#include <signal_context_slab.h>
 
 namespace Genode { class Signal_broker; }
 
@@ -23,13 +24,12 @@ class Genode::Signal_broker
 {
 	private:
 
-		Allocator                        &_md_alloc;
-		Rpc_entrypoint                   &_source_ep;
-		Rpc_entrypoint                   &_context_ep;
-		Signal_source_component           _source;
-		Signal_source_capability          _source_cap;
-		Tslab<Signal_context_component,
-		      960*sizeof(long)>           _contexts_slab { &_md_alloc };
+		Allocator               &_md_alloc;
+		Rpc_entrypoint          &_source_ep;
+		Rpc_entrypoint          &_context_ep;
+		Signal_source_component  _source;
+		Signal_source_capability _source_cap;
+		Signal_context_slab      _contexts_slab { _md_alloc };
 
 	public:
 
@@ -52,7 +52,7 @@ class Genode::Signal_broker
 			_source_ep.dissolve(&_source);
 
 			/* free all signal contexts */
-			while (Signal_context_component *r = _contexts_slab.first_object())
+			while (Signal_context_component *r = _contexts_slab.any_signal_context())
 				free_context(r->cap());
 		}
 

repos/base/src/core/include/signal_context_slab.h

@@ -0,0 +1,49 @@
+/*
+ * \brief  Slab allocator for signal contexts
+ * \author Norman Feske
+ * \date   2017-05-10
+ */
+
+/*
+ * Copyright (C) 2017 Genode Labs GmbH
+ *
+ * This file is part of the Genode OS framework, which is distributed
+ * under the terms of the GNU Affero General Public License version 3.
+ */
+
+#ifndef _CORE__INCLUDE__SIGNAL_CONTEXT_SLAB_H_
+#define _CORE__INCLUDE__SIGNAL_CONTEXT_SLAB_H_
+
+/* Genode includes */
+#include <base/slab.h>
+
+/* core-local includes */
+#include <signal_source_component.h>
+
+namespace Genode { struct Signal_context_slab; }
+
+
+/**
+ * Slab allocator for signal contexts
+ *
+ * We define an initial slab block to prevent a dynamic slab-block allocation
+ * via '_md_alloc' at the slab's construction time. This would be a problem
+ * because the '_md_alloc' supplied by the 'Pd_session_component' constructor
+ * uses the PD session itself as backing store (which would be in the middle of
+ * construction).
+ */
+struct Genode::Signal_context_slab : Slab
+{
+	static constexpr size_t SBS = 960*sizeof(long);
+	uint8_t _initial_sb[SBS];
+
+	Signal_context_slab(Allocator &md_alloc)
+	: Slab(sizeof(Signal_context_component), SBS, _initial_sb, &md_alloc) { }
+
+	Signal_context_component *any_signal_context()
+	{
+		return (Signal_context_component *)Slab::any_used_elem();
+	}
+};
+
+#endif /* _CORE__INCLUDE__SIGNAL_CONTEXT_SLAB_H_ */

repos/base/src/core/include/signal_source_component.h

@@ -29,6 +29,8 @@ namespace Genode {
 	class Signal_source_component;
 
 	typedef Fifo<Signal_context_component> Signal_queue;
+
+	struct Signal_context_slab;
 }