From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Fri, 30 Oct 2015 21:11:55 +0000 (-0400)
Subject: Fix: urcu-signal: smp_mb_master() needs registry lock
X-Git-Tag: v0.7.17~1
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=a554a662cc27aa360e514479693f41a3b69bbffa;p=urcu.git

Fix: urcu-signal: smp_mb_master() needs registry lock

The signal-based urcu flavor calls smp_mb_master() within the wait_gp()
function. Since commit "Fix: deadlock when thread join is issued in
read-side C.S.", wait_gp() is called without the registry lock held.

Ensure that the registry lock is only released around the wait per se,
not around the call to smp_mb_master(), otherwise we end up iterating on
a non-consistent thread registry in smp_mb_master().

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---

diff --git a/urcu.c b/urcu.c
index c5abeaf..e9df01b 100644
--- a/urcu.c
+++ b/urcu.c
@@ -219,19 +219,27 @@ static void smp_mb_master(int group)
 
 /*
  * synchronize_rcu() waiting. Single thread.
+ * Always called with rcu_registry lock held. Releases this lock and
+ * grabs it again. Holds the lock when it returns.
  */
 static void wait_gp(void)
 {
-	/* Read reader_gp before read futex */
+	/*
+	 * Read reader_gp before read futex. smp_mb_master() needs to
+	 * be called with the rcu registry lock held in RCU_SIGNAL
+	 * flavor.
+	 */
 	smp_mb_master(RCU_MB_GROUP);
+	/* Temporarily unlock the registry lock. */
+	mutex_unlock(&rcu_registry_lock);
 	if (uatomic_read(&gp_futex) != -1)
-		return;
+		goto end;
 	while (futex_async(&gp_futex, FUTEX_WAIT, -1,
 			NULL, NULL, 0)) {
 		switch (errno) {
 		case EWOULDBLOCK:
 			/* Value already changed. */
-			return;
+			goto end;
 		case EINTR:
 			/* Retry if interrupted by signal. */
 			break;	/* Get out of switch. */
@@ -240,6 +248,11 @@ static void wait_gp(void)
 			urcu_die(errno);
 		}
 	}
+end:
+	/*
+	 * Re-lock the registry lock before the next loop.
+	 */
+	mutex_lock(&rcu_registry_lock);
 }
 
 /*
@@ -300,14 +313,19 @@ void update_counter_and_wait(void)
 			}
 			break;
 		} else {
-			/* Temporarily unlock the registry lock. */
-			mutex_unlock(&rcu_registry_lock);
-			if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS)
+			if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
+				/* wait_gp unlocks/locks registry lock. */
 				wait_gp();
-			else
+			} else {
+				/* Temporarily unlock the registry lock. */
+				mutex_unlock(&rcu_registry_lock);
 				caa_cpu_relax();
-			/* Re-lock the registry lock before the next loop. */
-			mutex_lock(&rcu_registry_lock);
+				/*
+				 * Re-lock the registry lock before the
+				 * next loop.
+				 */
+				mutex_lock(&rcu_registry_lock);
+			}
 		}
 #else /* #ifndef HAS_INCOHERENT_CACHES */
 		/*
@@ -327,16 +345,20 @@ void update_counter_and_wait(void)
 				smp_mb_master(RCU_MB_GROUP);
 				wait_gp_loops = 0;
 			}
-			/* Temporarily unlock the registry lock. */
-			mutex_unlock(&rcu_registry_lock);
 			if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
+				/* wait_gp unlocks/locks registry lock. */
 				wait_gp();
 				wait_gp_loops++;
 			} else {
+				/* Temporarily unlock the registry lock. */
+				mutex_unlock(&rcu_registry_lock);
 				caa_cpu_relax();
+				/*
+				 * Re-lock the registry lock before the
+				 * next loop.
+				 */
+				mutex_lock(&rcu_registry_lock);
 			}
-			/* Re-lock the registry lock before the next loop. */
-			mutex_lock(&rcu_registry_lock);
 		}
 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
 	}