ircd::ctx: Proper condition variable exclusion semantics.

include/ircd/ctx/condition_variable.h

@@ -16,11 +16,16 @@ namespace ircd::ctx
 	struct condition_variable;
 }
 
-struct ircd::ctx::condition_variable
+class ircd::ctx::condition_variable
 {
-	dock d;
+	list q;
+
+	void notify(ctx &) noexcept;
 
   public:
+	bool empty() const;
+	size_t size() const;
+
 	template<class lock, class time_point, class predicate> bool wait_until(lock &, time_point&& tp, predicate&& pred);
 	template<class lock, class time_point> std::cv_status wait_until(lock &, time_point&& tp);
 
@@ -35,33 +40,59 @@ struct ircd::ctx::condition_variable
 	void notify() noexcept;
 };
 
+/// Wake up the next context waiting on the condition_variable
+///
+/// Unlike notify_one(), the next context in the queue is repositioned in the
+/// back before being woken up for fairness.
 inline void
 ircd::ctx::condition_variable::notify()
 noexcept
 {
-	d.notify();
+	ctx *c;
+	if(!(c = q.pop_front()))
+		return;
+
+	q.push_back(c);
+	notify(*c);
 }
 
+/// Wake up the next context waiting on the condition_variable
 inline void
 ircd::ctx::condition_variable::notify_one()
 noexcept
 {
-	d.notify_one();
+	if(!q.empty())
+		notify(*q.front());
 }
 
+/// Wake up all contexts waiting on the condition_variable.
+///
+/// We post all notifications without requesting direct context
+/// switches. This ensures everyone gets notified in a single
+/// transaction without any interleaving during this process.
 inline void
 ircd::ctx::condition_variable::notify_all()
 noexcept
 {
-	d.notify_all();
+	q.for_each([](ctx &c)
+	{
+		ircd::ctx::notify(c);
+	});
 }
 
 template<class lock>
 void
 ircd::ctx::condition_variable::wait(lock &l)
 {
-	const unlock_guard<lock> ul(l);
+	assert(current);
-	d.wait();
+	const unwind remove{[this]
+	{
+		q.remove(current);
+	}};
+
+	q.push_back(current);
+	const unlock_guard<lock> ul{l};
+	ircd::ctx::wait();
 }
 
 template<class lock,
@@ -70,31 +101,46 @@ void
 ircd::ctx::condition_variable::wait(lock &l,
                                     predicate&& pred)
 {
-	const unlock_guard<lock> ul(l);
+	if(pred())
-	return d.wait([&l, &pred]
+		return;
-	{
-		l.lock();
-		const unwind ul{[&l]
-		{
-			l.unlock();
-		}};
 
-		return pred();
+	assert(current);
-	});
+	const unwind remove{[this]
+	{
+		q.remove(current);
+	}};
+
+	q.push_back(current); do
+	{
+		const unlock_guard<lock> ul{l};
+		ircd::ctx::wait();
+	}
+	while(!pred());
 }
 
+/// Returns true if notified; false if timed out
 template<class lock,
          class duration>
 std::cv_status
 ircd::ctx::condition_variable::wait_for(lock &l,
                                         const duration &dur)
 {
-	const unlock_guard<lock> ul(l);
+	static const duration zero(0);
-	return d.wait_for(dur)?
+
+	assert(current);
+	const unwind remove{[this]
+	{
+		q.remove(current);
+	}};
+
+	q.push_back(current);
+	const unlock_guard<lock> ul{l};
+	return ircd::ctx::wait<std::nothrow_t>(dur) > zero?
 		std::cv_status::no_timeout:
 		std::cv_status::timeout;
 }
 
+/// Returns true if predicate passed; false if timed out
 template<class lock,
          class duration,
          class predicate>
@@ -103,31 +149,55 @@ ircd::ctx::condition_variable::wait_for(lock &l,
                                         const duration &dur,
                                         predicate&& pred)
 {
-	const unlock_guard<lock> ul(l);
+	static const duration zero(0);
-	return d.wait_for(dur, [&l, &pred]
-	{
-		l.lock();
-		const unwind ul{[&l]
-		{
-			l.unlock();
-		}};
 
-		return pred();
+	if(pred())
-	});
+		return true;
+
+	assert(current);
+	const unwind remove{[this]
+	{
+		q.remove(current);
+	}};
+
+	q.push_back(current); do
+	{
+		bool expired;
+		{
+			const unlock_guard<lock> ul{l};
+			expired = ircd::ctx::wait<std::nothrow_t>(dur) <= zero;
+		};
+
+		if(pred())
+			return true;
+
+		if(expired)
+			return false;
+	}
+	while(1);
 }
 
+/// Returns true if notified; false if timed out
 template<class lock,
          class time_point>
 std::cv_status
 ircd::ctx::condition_variable::wait_until(lock &l,
                                           time_point&& tp)
 {
-	const unlock_guard<lock> ul(l);
+	assert(current);
-	return d.wait_until(std::forward<time_point>(tp))?
+	const unwind remove{[this]
-		std::cv_status::no_timeout:
+	{
-		std::cv_status::timeout;
+		q.remove(current);
+	}};
+
+	q.push_back(current);
+	const unlock_guard<lock> ul{l};
+	return ircd::ctx::wait_until<std::nothrow_t>(tp)?
+		std::cv_status::timeout:
+		std::cv_status::no_timeout;
 }
 
+/// Returns true if predicate passed; false if timed out
 template<class lock,
          class time_point,
          class predicate>
@@ -136,15 +206,58 @@ ircd::ctx::condition_variable::wait_until(lock &l,
                                           time_point&& tp,
                                           predicate&& pred)
 {
-	const unlock_guard<lock> ul(l);
+	if(pred())
-	return d.wait_until(std::forward<time_point>(tp), [&l, &pred]
+		return true;
-	{
-		l.lock();
-		const unwind ul{[&l]
-		{
-			l.unlock();
-		}};
 
-		return pred();
+	assert(current);
-	});
+	const unwind remove{[this]
+	{
+		q.remove(current);
+	}};
+
+	q.push_back(current); do
+	{
+		bool expired;
+		{
+			const unlock_guard<lock> ul{l};
+			expired = ircd::ctx::wait_until<std::nothrow_t>(tp);
+		};
+
+		if(pred())
+			return true;
+
+		if(expired)
+			return false;
+	}
+	while(1);
+}
+
+inline void
+ircd::ctx::condition_variable::notify(ctx &ctx)
+noexcept
+{
+	// This branch handles condition_variable.notify() being called from outside
+	// the context system. If a context is currently running we can make a direct
+	// context-switch with yield(ctx), otherwise notify(ctx) enqueues the context.
+
+	if(current)
+		ircd::ctx::yield(ctx);
+	else
+		ircd::ctx::notify(ctx);
+}
+
+/// The number of contexts waiting in the queue.
+inline size_t
+ircd::ctx::condition_variable::size()
+const
+{
+	return q.size();
+}
+
+/// The number of contexts waiting in the queue.
+inline bool
+ircd::ctx::condition_variable::empty()
+const
+{
+	return q.empty();
 }