From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Thu, 3 Mar 2011 16:46:20 +0000 (-0500)
Subject: fork: child should issue synchronize_rcu() for urcu-bp garbage collection
X-Git-Tag: v1.9.1~435
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=0b362d6f999cbbe4b1a0d8cee2a9fb9a34422b7a;p=lttng-ust.git

fork: child should issue synchronize_rcu() for urcu-bp garbage collection

After a fork(), the child process should execute synchronize_rcu() before any
new thread can be created. Failure to do so could lead to a deadlock in the
unlikely scenario where a thread ID appearing in the parent is reused in the
child before GC is performed.

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

diff --git a/libust/tracectl.c b/libust/tracectl.c
index bef4537d..3b609753 100644
--- a/libust/tracectl.c
+++ b/libust/tracectl.c
@@ -1711,7 +1711,7 @@ static void ust_after_fork_common(ust_fork_info_t *fork_info)
 
 void ust_after_fork_parent(ust_fork_info_t *fork_info)
 {
-	/* Reenable signals */
+	/* Release mutexes and reenable signals */
 	ust_after_fork_common(fork_info);
 }
 
@@ -1720,7 +1720,15 @@ void ust_after_fork_child(ust_fork_info_t *fork_info)
 	/* First sanitize the child */
 	ust_fork();
 
-	/* Then reenable interrupts */
+	/* Then release mutexes and reenable signals */
 	ust_after_fork_common(fork_info);
+
+	/*
+	 * Make sure we clean up the urcu-bp thread list in the child by running
+	 * the garbage collection before any pthread_create can be called.
+	 * Failure to do so could lead to a deadlock caused by reuse of a thread
+	 * ID before urcu-bp garbage collection is performed.
+	 */
+	synchronize_rcu();
 }