From 40e140c9f237bbe8c50d4a5355561139f01afb70 Mon Sep 17 00:00:00 2001
From: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Date: Thu, 12 Feb 2009 01:00:00 -0500
Subject: [PATCH] Add barriers

Use smp_rmb() is busy-loops to make sure we re-read the variable.
Kick the reader threads we are waiting for after a few loops.
Documents memory ordering in rcu_read_lock().

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
---
 urcu.c | 43 ++++++++++++++++++++++++++++++++++++++-----
 urcu.h |  9 +++++++++
 2 files changed, 47 insertions(+), 5 deletions(-)

diff --git a/urcu.c b/urcu.c
index 018e09b..0e42207 100644
--- a/urcu.c
+++ b/urcu.c
@@ -77,11 +77,32 @@ static void switch_next_urcu_qparity(void)
 }
 
 #ifdef DEBUG_FULL_MB
+static void force_mb_single_thread(pthread_t tid)
+{
+	smp_mb();
+}
+
 static void force_mb_all_threads(void)
 {
 	smp_mb();
 }
 #else
+
+static void force_mb_single_thread(pthread_t tid)
+{
+	assert(reader_data);
+	sig_done = 0;
+	smp_mb();	/* write sig_done before sending the signals */
+	pthread_kill(tid, SIGURCU);
+	/*
+	 * Wait for sighandler (and thus mb()) to execute on every thread.
+	 * BUSY-LOOP.
+	 */
+	while (sig_done < 1)
+		smp_rmb();	/* ensure we re-read sig-done */
+	smp_mb();	/* read sig_done before ending the barrier */
+}
+
 static void force_mb_all_threads(void)
 {
 	struct reader_data *index;
@@ -100,7 +121,7 @@ static void force_mb_all_threads(void)
 	 * BUSY-LOOP.
 	 */
 	while (sig_done < num_readers)
-		barrier();
+		smp_rmb();	/* ensure we re-read sig-done */
 	smp_mb();	/* read sig_done before ending the barrier */
 }
 #endif
@@ -111,14 +132,21 @@ void wait_for_quiescent_state(void)
 
 	if (!reader_data)
 		return;
-	/* Wait for each thread urcu_active_readers count to become 0.
+	/*
+	 * Wait for each thread urcu_active_readers count to become 0.
 	 */
 	for (index = reader_data; index < reader_data + num_readers; index++) {
+		int wait_loops = 0;
 		/*
-		 * BUSY-LOOP.
+		 * BUSY-LOOP. Force the reader thread to commit its
+		 * urcu_active_readers update to memory if we wait for too long.
 		 */
-		while (rcu_old_gp_ongoing(index->urcu_active_readers))
-			barrier();
+		while (rcu_old_gp_ongoing(index->urcu_active_readers)) {
+			if (wait_loops++ == KICK_READER_LOOPS) {
+				force_mb_single_thread(index->tid);
+				wait_loops = 0;
+			}
+		}
 	}
 }
 
@@ -243,6 +271,11 @@ void urcu_unregister_thread(void)
 #ifndef DEBUG_FULL_MB
 void sigurcu_handler(int signo, siginfo_t *siginfo, void *context)
 {
+	/*
+	 * Executing this smp_mb() is the only purpose of this signal handler.
+	 * It punctually promotes barrier() into smp_mb() on every thread it is
+	 * executed on.
+	 */
 	smp_mb();
 	atomic_inc(&sig_done);
 }
diff --git a/urcu.h b/urcu.h
index 1b663c7..3eca5ea 100644
--- a/urcu.h
+++ b/urcu.h
@@ -130,6 +130,13 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr,
 
 #define SIGURCU SIGUSR1
 
+/*
+ * If a reader is really non-cooperative and refuses to commit its
+ * urcu_active_readers count to memory (there is no barrier in the reader
+ * per-se), kick it after a few loops waiting for it.
+ */
+#define KICK_READER_LOOPS 10000
+
 #ifdef DEBUG_YIELD
 #include <sched.h>
 #include <time.h>
@@ -234,6 +241,8 @@ static inline void rcu_read_lock(void)
 
 	tmp = urcu_active_readers;
 	/* urcu_gp_ctr = RCU_GP_COUNT | (~RCU_GP_CTR_BIT or RCU_GP_CTR_BIT) */
+	/* The data dependency "read urcu_gp_ctr, write urcu_active_readers",
+	 * serializes those two memory operations. */
 	if (likely(!(tmp & RCU_GP_CTR_NEST_MASK)))
 		urcu_active_readers = urcu_gp_ctr;
 	else
-- 
2.34.1