bit removed = 0;
bit free = 0;

#define RCU_GP_CTR_BIT (1 << 7)
#define RCU_GP_CTR_NEST_MASK (RCU_GP_CTR_BIT - 1)

bit need_mb = 0;
byte urcu_gp_ctr = 1;
byte urcu_active_readers = 0;

bit reader_progress[4];

proctype urcu_reader()
{
	bit done = 0;
	byte tmp;
	byte tmp_removed;
	byte tmp_free;

	do
	:: 1 ->
		if
		:: need_mb == 1 ->
			need_mb = 0;
		:: else -> break;
		fi
	od;
	do
	:: 1 ->
		if
		:: reader_progress[0] == 0 ->
			tmp = urcu_active_readers;
			if
			:: (tmp & RCU_GP_CTR_NEST_MASK) == 0 ->
				urcu_active_readers = urcu_gp_ctr;
			:: else ->
				urcu_active_readers = tmp + 1;
			fi;
			reader_progress[0] = 1;
		:: reader_progress[1] == 0 ->
			tmp_removed = removed;
			reader_progress[1] = 1;
		:: reader_progress[2] == 0 ->
			tmp_free = free;
			reader_progress[2] = 1;
		:: ((reader_progress[0] == 1) && (reader_progress[3] == 0)) ->
			urcu_active_readers = urcu_active_readers - 1;
		:: else -> break;
		fi;
		atomic {
			tmp = 0;
			do
			:: reader_progress[tmp] == 0 ->
				tmp = tmp + 1;
				break;
			:: reader_progress[tmp] == 1 && tmp < 4 ->
				tmp = tmp + 1;
			:: tmp >= 4 ->
				done = 1;
				break;
			od;
			do
			:: tmp < 4 && reader_progress[tmp] == 0 ->
				tmp = tmp + 1;
			:: tmp < 4 && reader_progress[tmp] == 1 ->
				break;
			:: tmp >= 4 ->
				if
				:: need_mb == 1 ->
					need_mb = 0;
				:: else -> skip;
				fi;
				done = 1;
				break;
			od

		}
		if
		:: done == 1 -> break;
		:: else -> skip;
		fi
	od;
	do
	:: 1 ->
		if
		:: need_mb == 1 ->
			need_mb = 0;
		:: else -> skip;
		fi;
		assert((free == 0) || (removed == 1));
	od;
}

proctype urcu_updater()
{
	removed = 1;
	need_mb = 1;
	do
	:: need_mb == 1 -> skip;
	:: need_mb == 0 -> break;
	od;
	urcu_gp_ctr = urcu_gp_ctr + RCU_GP_CTR_BIT;
	do
	:: 1 ->
		if
		:: (urcu_active_readers & RCU_GP_CTR_NEST_MASK) != 0 &&
		   (urcu_active_readers & ~RCU_GP_CTR_NEST_MASK) !=
		   (urcu_gp_ctr & ~RCU_GP_CTR_NEST_MASK) ->
			skip;
		:: else -> break;
		fi
	od;

	need_mb = 1;
	do
	:: need_mb == 1 -> skip;
	:: need_mb == 0 -> break;
	od;
	urcu_gp_ctr = urcu_gp_ctr + RCU_GP_CTR_BIT;
	do
	:: 1 ->
		if
		:: (urcu_active_readers & RCU_GP_CTR_NEST_MASK) != 0 &&
		   (urcu_active_readers & ~RCU_GP_CTR_NEST_MASK) !=
		   (urcu_gp_ctr & ~RCU_GP_CTR_NEST_MASK) ->
			skip;
		:: else -> break;
		fi;
	od;

	free = 1;
}

init {
	atomic {
		reader_progress[0] = 0;
		reader_progress[1] = 0;
		reader_progress[2] = 0;
		reader_progress[3] = 0;
		run urcu_reader();
		run urcu_updater();
	}
}