From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Mon, 30 Sep 2013 15:49:32 +0000 (-0400)
Subject: Fix: urcu-bp: Bulletproof RCU arena resize bug
X-Git-Tag: v0.9.0~141
X-Git-Url: https://git.lttng.org/?p=urcu.git;a=commitdiff_plain;h=9d8612b71148c42a430e1419bad3b9b09453f64a

Fix: urcu-bp: Bulletproof RCU arena resize bug

> From: "Milosz Tanski" <milosz@adfin.com>

> While trying to use the BP flavor of RCU I ran into random crashes. I
> tracked it down to issues with resizing of the BP RCU memory pool.
>
> The problem is in the urcu-bp.c file in the resize_arena() function.
> On successful allocation / remapping the len member of the
> registry_arena struct is never set anywhere function. On the second
> resize of the arena the code in resize_arena() still thinks the
> previous size is equal to the original mapping size. I've fixed this
> issue locally by just adding the following code at the bottom of
> resize_arena().

Good catch !!

However, I think your fix misses one case: if we happen to re-use the
same region, we want to update the length too.

Reported-by: Milosz Tanski <milosz@adfin.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---

diff --git a/urcu-bp.c b/urcu-bp.c
index a823659..93d781f 100644
--- a/urcu-bp.c
+++ b/urcu-bp.c
@@ -305,26 +305,37 @@ int rcu_read_ongoing(void)
  */
 static void resize_arena(struct registry_arena *arena, size_t len)
 {
-	void *new_arena;
+	void *new_p;
+	size_t old_len;
+
+	old_len = arena->len;
 
 	if (!arena->p)
-		new_arena = mmap(arena->p, len,
-				 PROT_READ | PROT_WRITE,
-				 MAP_ANONYMOUS | MAP_PRIVATE,
-				 -1, 0);
+		new_p = mmap(arena->p, len,
+			PROT_READ | PROT_WRITE,
+			MAP_ANONYMOUS | MAP_PRIVATE,
+			-1, 0);
 	else
-		new_arena = mremap_wrapper(arena->p, arena->len,
-				   	len, MREMAP_MAYMOVE);
-	assert(new_arena != MAP_FAILED);
+		new_p = mremap_wrapper(arena->p, old_len,
+			len, MREMAP_MAYMOVE);
+	assert(new_p != MAP_FAILED);
+
+	/*
+	 * Zero the newly allocated memory. Since mmap() does not
+	 * clearly specify if memory is zeroed or not (although it is
+	 * very likely that it is), be extra careful by not expecting
+	 * the new range to be zeroed by mremap.
+	 */
+	bzero(new_p + old_len, len - old_len);
 
 	/*
-	 * re-used the same region ?
+	 * If we did not re-use the same region, we need to update the
+	 * arena pointer.
 	 */
-	if (new_arena == arena->p)
-		return;
+	if (new_p != arena->p)
+		arena->p = new_p;
 
-	bzero(new_arena + arena->len, len - arena->len);
-	arena->p = new_arena;
+	arena->len = len;
 }
 
 /* Called with signals off and mutex locked */