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From: Jason Evans <jasone@FreeBSD.org>
Date: Mon, 1 Dec 2008 10:20:59 +0000 (UTC)
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Subject: svn commit: r185514 - head/lib/libc/stdlib
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Author: jasone
Date: Mon Dec  1 10:20:59 2008
New Revision: 185514
URL: http://svn.freebsd.org/changeset/base/185514

Log:
  Fix a lock order reversal bug that could cause deadlock during fork(2).
  
  Reported by:	kib

Modified:
  head/lib/libc/stdlib/malloc.c

Modified: head/lib/libc/stdlib/malloc.c
==============================================================================
--- head/lib/libc/stdlib/malloc.c	Mon Dec  1 10:16:25 2008	(r185513)
+++ head/lib/libc/stdlib/malloc.c	Mon Dec  1 10:20:59 2008	(r185514)
@@ -5515,16 +5515,41 @@ _malloc_thread_cleanup(void)
 void
 _malloc_prefork(void)
 {
-	unsigned i;
+	bool again;
+	unsigned i, j;
+	arena_t *larenas[narenas], *tarenas[narenas];
 
 	/* Acquire all mutexes in a safe order. */
 
-	malloc_spin_lock(&arenas_lock);
-	for (i = 0; i < narenas; i++) {
-		if (arenas[i] != NULL)
-			malloc_spin_lock(&arenas[i]->lock);
-	}
-	malloc_spin_unlock(&arenas_lock);
+	/*
+	 * arenas_lock must be acquired after all of the arena mutexes, in
+	 * order to avoid potential deadlock with arena_lock_balance[_hard]().
+	 * Since arenas_lock protects the arenas array, the following code has
+	 * to race with arenas_extend() callers until it succeeds in locking
+	 * all arenas before locking arenas_lock.
+	 */
+	memset(larenas, 0, sizeof(arena_t *) * narenas);
+	do {
+		again = false;
+
+		malloc_spin_lock(&arenas_lock);
+		for (i = 0; i < narenas; i++) {
+			if (arenas[i] != larenas[i]) {
+				memcpy(tarenas, arenas, sizeof(arena_t *) *
+				    narenas);
+				malloc_spin_unlock(&arenas_lock);
+				for (j = 0; j < narenas; j++) {
+					if (larenas[j] != tarenas[j]) {
+						larenas[j] = tarenas[j];
+						malloc_spin_lock(
+						    &larenas[j]->lock);
+					}
+				}
+				again = true;
+				break;
+			}
+		}
+	} while (again);
 
 	malloc_mutex_lock(&base_mtx);
 
@@ -5539,6 +5564,7 @@ void
 _malloc_postfork(void)
 {
 	unsigned i;
+	arena_t *larenas[narenas];
 
 	/* Release all mutexes, now that fork() has completed. */
 
@@ -5550,12 +5576,12 @@ _malloc_postfork(void)
 
 	malloc_mutex_unlock(&base_mtx);
 
-	malloc_spin_lock(&arenas_lock);
+	memcpy(larenas, arenas, sizeof(arena_t *) * narenas);
+	malloc_spin_unlock(&arenas_lock);
 	for (i = 0; i < narenas; i++) {
-		if (arenas[i] != NULL)
-			malloc_spin_unlock(&arenas[i]->lock);
+		if (larenas[i] != NULL)
+			malloc_spin_unlock(&larenas[i]->lock);
 	}
-	malloc_spin_unlock(&arenas_lock);
 }
 
 /*