fix init race that could lead to deadlock in malloc init code

the malloc init code provided its own version of pthread_once type
logic, including the exact same bug that was fixed in pthread_once in
commit 0d0c2f40344640a2a6942dda156509593f51db5d.

since this code is called adjacent to expand_heap, which takes a lock,
there is no reason to have pthread_once-type initialization. simply
moving the init code into the interval where expand_heap already holds
its lock on the brk achieves the same result with much less
synchronization logic, and allows the buggy code to be eliminated
rather than just fixed.

1 files changed, 14 insertions, 39 deletions

diff --git a/src/malloc/malloc.c b/src/malloc/malloc.c
index 70c7b3f..4f61807 100644
--- a/src/malloc/malloc.c
+++ b/src/malloc/malloc.c
@@ -154,11 +154,22 @@ void __dump_heap(int x)
 
 static struct chunk *expand_heap(size_t n)
 {
+	static int init;
 	struct chunk *w;
 	uintptr_t new;
 
 	lock(mal.brk_lock);
 
+	if (!init) {
+		mal.brk = __brk(0);
+#ifdef SHARED
+		mal.brk = mal.brk + PAGE_SIZE-1 & -PAGE_SIZE;
+#endif
+		mal.brk = mal.brk + 2*SIZE_ALIGN-1 & -SIZE_ALIGN;
+		mal.heap = (void *)mal.brk;
+		init = 1;
+	}
+
 	if (n > SIZE_MAX - mal.brk - 2*PAGE_SIZE) goto fail;
 	new = mal.brk + n + SIZE_ALIGN + PAGE_SIZE - 1 & -PAGE_SIZE;
 	n = new - mal.brk;
@@ -186,6 +197,9 @@ static struct chunk *expand_heap(size_t n)
 		return area;
 	}
 
+	w = MEM_TO_CHUNK(mal.heap);
+	w->psize = 0 | C_INUSE;
+
 	w = MEM_TO_CHUNK(new);
 	w->psize = n | C_INUSE;
 	w->csize = 0 | C_INUSE;
@@ -203,44 +217,6 @@ fail:
 	return 0;
 }
 
-static int init_malloc(size_t n)
-{
-	static volatile int init, waiters;
-	int state;
-	struct chunk *c;
-
-	if (init == 2) return 0;
-
-	while ((state=a_swap(&init, 1)) == 1)
-		__wait(&init, &waiters, 1, 1);
-	if (state) {
-		a_store(&init, 2);
-		return 0;
-	}
-
-	mal.brk = __brk(0);
-#ifdef SHARED
-	mal.brk = mal.brk + PAGE_SIZE-1 & -PAGE_SIZE;
-#endif
-	mal.brk = mal.brk + 2*SIZE_ALIGN-1 & -SIZE_ALIGN;
-
-	c = expand_heap(n);
-
-	if (!c) {
-		a_store(&init, 0);
-		if (waiters) __wake(&init, 1, 1);
-		return -1;
-	}
-
-	mal.heap = (void *)c;
-	c->psize = 0 | C_INUSE;
-	free(CHUNK_TO_MEM(c));
-
-	a_store(&init, 2);
-	if (waiters) __wake(&init, -1, 1);
-	return 1;
-}
-
 static int adjust_size(size_t *n)
 {
 	/* Result of pointer difference must fit in ptrdiff_t. */
@@ -375,7 +351,6 @@ void *malloc(size_t n)
 	for (;;) {
 		uint64_t mask = mal.binmap & -(1ULL<<i);
 		if (!mask) {
-			if (init_malloc(n) > 0) continue;
 			c = expand_heap(n);
 			if (!c) return 0;
 			if (alloc_rev(c)) {