Issue #1699 - Part 3b: mozjemalloc: Bootstrap allocator, early diversion for FreeBSD

Coded a simple memory allocator meant to be used during jemalloc bootstrap (malloc_init_hard()). Although protected by "#ifdef __FreeBSD__", it is not FreeBSD-specific: Any POSIX platform could use it. Hook it so that it is used in place of jemalloc's own routines while malloc_init_hard() is executing or memory for mutexes is being allocated. Currently, 'malloc', 'calloc', '*memalign' and 'free' are diverted during init or lock initializations. Details are quite complex, see the big comment block starting with "There are several problematic interactions between FreeBSD's libthr and this jemalloc." for more explanations. Also replaced ad-hoc BSD code to determine the number of CPUs with 'sysconf', which is POSIX-compliant (and supported on modern BSDs).
2026-05-26 14:54:25 +00:00 · 2021-01-06 11:30:19 +01:00
parent 208a992fe9
commit b7e6f5fd45
1 changed files with 294 additions and 42 deletions
@@ -174,6 +174,9 @@
 #endif

 #include <sys/types.h>
+#ifdef MOZ_MEMORY_BSD
+#include <sys/sysctl.h>
+#endif

 #include <errno.h>
 #include <stdlib.h>
@@ -489,10 +492,10 @@ typedef pthread_mutex_t malloc_spinlock_t;
 #endif

 /* Set to true once the allocator has been initialized. */
-static bool malloc_initialized = false;
+static volatile bool malloc_initialized = false;

-#if defined(MOZ_MEMORY_WINDOWS)
-/* No init lock for Windows. */
+#if defined(MOZ_MEMORY_WINDOWS) || defined(__FreeBSD__)
+/* No init lock for Windows nor FreeBSD. */
 #elif defined(MOZ_MEMORY_DARWIN)
 static malloc_mutex_t init_lock = {OS_SPINLOCK_INIT};
 #elif defined(MOZ_MEMORY_LINUX)
@@ -1388,6 +1391,11 @@ void	(*_malloc_message)(const char *p1, const char *p2, const char *p3,
 * cases.
 */

+#ifdef __FreeBSD__
+// If true, memory calls must be diverted to the bootstrap allocator
+static __thread bool in_mutex_init = false;
+#endif
+
 static bool
 malloc_mutex_init(malloc_mutex_t *mutex)
 {
@@ -1406,6 +1414,19 @@ malloc_mutex_init(malloc_mutex_t *mutex)
 		return (true);
 	}
 	pthread_mutexattr_destroy(&attr);
+#elif defined(__FreeBSD__)
+	in_mutex_init = true;
+
+	*mutex = PTHREAD_ADAPTIVE_MUTEX_INITIALIZER_NP;
+
+	// Make sure necessary mutex memory is allocated right now, with
+	// 'in_mutex_init' set to true (allocations to be diverted to the
+	// bootstrap allocator). Also force multi-thread initialization in
+	// libthr (checked and performed in 'pthread_mutex_lock').
+	pthread_mutex_lock(mutex);
+	pthread_mutex_unlock(mutex);
+
+	in_mutex_init = false;
 #else
 	if (pthread_mutex_init(mutex, NULL) != 0)
 		return (true);
@@ -1460,6 +1481,8 @@ malloc_spin_init(malloc_spinlock_t *lock)
 		return (true);
 	}
 	pthread_mutexattr_destroy(&attr);
+#elif defined(__FreeBSD__)
+	malloc_lock_init(lock);
 #else
 	if (pthread_mutex_init(lock, NULL) != 0)
 		return (true);
@@ -4744,25 +4767,7 @@ huge_dalloc(void *ptr)
 * Platform-specific methods to determine the number of CPUs in a system.
 * This will be used to determine the desired number of arenas.
 */
-#ifdef MOZ_MEMORY_BSD
-static inline unsigned
-malloc_ncpus(void)
-{
-	unsigned ret;
-	int mib[2];
-	size_t len;
-
-	mib[0] = CTL_HW;
-	mib[1] = HW_NCPU;
-	len = sizeof(ret);
-	if (sysctl(mib, 2, &ret, &len, (void *) 0, 0) == -1) {
-		/* Error. */
-		return (1);
-	}
-
-	return (ret);
-}
-#elif (defined(MOZ_MEMORY_LINUX))
+#if (defined(MOZ_MEMORY_LINUX))
 #include <fcntl.h>

 static inline unsigned
@@ -4834,8 +4839,7 @@ malloc_ncpus(void)
 	else
 		return (n);
 }
-#elif (defined(MOZ_MEMORY_SOLARIS))
-
+#elif (defined(MOZ_MEMORY_SOLARIS) || defined(MOZ_MEMORY_BSD))
 static inline unsigned
 malloc_ncpus(void)
 {
@@ -4980,18 +4984,13 @@ malloc_print_stats(void)
 	}
 }

-/*
- * FreeBSD's pthreads implementation calls malloc(3), so the malloc
- * implementation has to take pains to avoid infinite recursion during
- * initialization.
- */
+
 #if (defined(MOZ_MEMORY_WINDOWS) || defined(MOZ_MEMORY_DARWIN))
 #define	malloc_init() false
 #else
 static inline bool
 malloc_init(void)
 {
-
 	if (malloc_initialized == false)
 		return (malloc_init_hard());

@@ -4999,6 +4998,238 @@ malloc_init(void)
 }
 #endif

+
+#ifdef __FreeBSD__
+// There are several problematic interactions between FreeBSD's libthr and this
+// jemalloc.
+//
+// 1. This malloc calls pthread_mutex_init at init, but in libthr this triggers
+// an allocation, causing an infinite recursion.
+// 2. Actually, this malloc assumes that lock initialization never triggers a
+// memory allocation, even after initialization (see 'arena_new').
+// 3. First use of a lock routine ('pthread_mutex_lock') in libthr triggers
+// initialization of the process as a multi-threaded process. Unfortunately,
+// libthr calls regular malloc as part of this bootstrap process.
+//
+// If there was no problem 3, we could have resolved this easily by using
+// constant mutex initializers, since then libthr's uses its own internal
+// allocator instead of regular malloc (this appears to have been the case for
+// years now). However, problem 3 requires this malloc to provide some memory
+// at places where it is not able to, so we need a way to divert standard
+// allocator functions to some simple bootstrap allocator. And once we have
+// done this, using constant mutex initializers looses most of its appeal,
+// because allocations for problems 1 & 2 can be fulfilled by the simple
+// allocator as well, without the drawback of being dependent on libthr's
+// specific behavior.
+//
+// Since the init lock controls the 'malloc_initialized' flag, it is not
+// possible to reliably check whether jemalloc is initialized in the case of
+// multiple threads with the given tools (pthread cannot be used yet, but
+// mutual exclusion is required). One solution would be to code simple
+// user-space locks for this (e.g., spinlocks using GCC's builtins). But an
+// even "simpler" solution is in fact to just remove the lock, on the ground
+// that there must be some memory allocation before multithreading is enabled,
+// so jemalloc is in fact always initialized before that point. And if there
+// is not, we'll provoke it.
+//
+// At some point, I implemented a solution using __constructor__, as
+// 'jemalloc_darwin_init', and tweaked the build so that it is included in
+// executables (in platform/build/gecko_templates.mozbuild). But this was not
+// enough: Clearly it could happen that some other library would be initialized
+// before jemalloc, calling malloc in its contructor. Could have tried to work
+// around this with constructor priorities, but this seemed fragile as well. So
+// in the end, I kept the calls to 'malloc_init' from the interface's
+// functions, and had to introduce 'malloc_initializing' to know when (part of
+// the) calls should be diverted. I finally kept the constructor as well, just
+// to absolutely guarantee that jemalloc is initialized during executable load,
+// that is to say, before multi-threading happens, in case initialization in
+// libthr or glib is removed at some point. It just doesn't call
+// 'malloc_init_hard', contrary to Darwin's, but 'malloc_init' (because
+// jemalloc normally has already been initialized at this point).
+//
+// During lock initialization, malloc is temporarily diverted to the bootstrap
+// allocator to avoid harmful recursion. This is achieved using a flag
+// indicating whether lock initialization is under way (in order to work also
+// after malloc_init_hard() has completed). The flag *must* be per-thread,
+// because creation of new arenas, which causes creation of new locks, can
+// happen at unpredictable moments after multi-threading has been enabled (and
+// malloc has been initialized), which means concurrent allocation requests can
+// occur, and must not all be diverted. With this flag in place, and an
+// additional change to ensure that libthr's multi-thread init is indeed done
+// during mutex init (through 'pthread_lock_mutex'), there was no need to keep
+// the 'malloc_initializing' flag (see previous paragraph).
+//
+// The most likely change this whole architecture is not immune to would be if
+// jemalloc starts initializing new locks after malloc_init_hard() has finished
+// but not under an existing lock (new arena's lock is currently initialized
+// under the arenas lock), because bootstrap allocator functions are not
+// thread-safe per se. If this happens, then a very simple spinlock
+// implementation on top of GCC's atomics will be in order. But I don't think
+// this is very likely to happen.
+
+// Diverts key (de)allocation functions when jemalloc's mutexes are
+// initializing (malloc_init_hard(), but also arena_new() and
+// malloc_rtree_new(), as of this writing).
+#define BA_DIVERT(code)						\
+	do {							\
+		if (in_mutex_init) {				\
+			code;					\
+		}						\
+	} while (0)
+
+
+// Bootstrap allocator
+//
+// It is not FreeBSD-specific, and could be used by any POSIX-compliant
+// platform if needed.
+//
+// Allocates one page at a time (relies on 'pagesize' as defined above in this
+// file), and returns memory from it. Does not accept allocations larger than a
+// single page (minus alignment). Will waste space at end of pages. Never frees
+// memory.
+//
+// All these constraints are not a problem, since this allocator is meant to
+// serve only some requests at initialization (no more than a few kB).
+
+// Number of really allocated bytes
+static size_t ba_allocated_bn = 0;
+
+// Number of requested bytes
+static size_t ba_requested_bn = 0;
+
+// Current address we are allocating from, or NULL if a new page has to be
+// allocated.
+static void *ba_cur_free = NULL;
+
+
+static void ba_alloc_new_page()
+{
+	ba_cur_free = mmap(NULL, pagesize, PROT_READ | PROT_WRITE,
+			   MAP_ANON | MAP_PRIVATE, -1, 0);
+	if (ba_cur_free == MAP_FAILED)
+		abort();
+
+	ba_allocated_bn += pagesize;
+}
+
+// Returns the offset to point to have point a multiple of alignment
+static size_t
+ba_offset_to_aligned(uintptr_t point, size_t alignment) {
+	if (alignment != 0) {
+		size_t rest = point % alignment;
+
+		if (rest != 0)
+			return alignment - rest;
+	}
+
+	return 0;
+}
+
+static void * ba_memalign(size_t alignment, size_t size)
+{
+	// We don't care about alignment being a power of 2, nor pagesize. Code
+	// below supports everything, provided that alignment divides the page
+	// size.
+
+	// Impose cache-line size minimum alignment, so that there is no cache
+	// trashing between fundamental structures.
+	if (alignment < CACHELINE)
+		alignment = CACHELINE;
+
+	if (size > pagesize ||
+	    alignment > pagesize ||
+	    size + alignment > pagesize ||
+	    pagesize % alignment != 0)
+		abort();
+
+	// Address to be returned
+	uintptr_t cur_free;
+
+	// Allocate a new page if no current page (startup or previous one was
+	// exhausted) or there is not enough remaining space in it.
+
+	if (ba_cur_free == NULL) {
+		// No current page
+		ba_alloc_new_page();
+		cur_free = (uintptr_t)ba_cur_free;
+	} else {
+		cur_free = (uintptr_t)ba_cur_free;
+
+		uintptr_t off = cur_free % pagesize;
+		uintptr_t al_off = ba_offset_to_aligned(off, alignment);
+
+		if (off + al_off + size > pagesize) {
+			// Not enough room. Need a new page.
+			ba_alloc_new_page();
+			cur_free = (uintptr_t)ba_cur_free;
+		} else
+			// Account for alignment
+			cur_free += al_off;
+	}
+
+	// Compute the next free address
+	uintptr_t next_free = cur_free + size;
+	if (next_free % pagesize == 0 && size != 0)
+		next_free = 0;
+
+	// Set it
+	ba_cur_free = (void *)next_free;
+
+	// Stats
+	ba_requested_bn += size;
+
+	// Done
+	return (void *)cur_free;
+}
+
+static void * ba_malloc(size_t size)
+{
+	// 64-bit alignment by default. ba_memalign imposes an even greater
+	// alignment anyway.
+	return ba_memalign(8, size);
+}
+
+static void * ba_calloc(size_t number, size_t size)
+{
+	size_t const bn = number * size;
+
+	if ((bn < number || bn < size) && bn != 0)
+		// Overflow
+		abort();
+
+	void * const res = ba_malloc(bn);
+	memset(res, 0, bn);
+	return res;
+}
+
+static void ba_free(void * ptr) {
+#ifdef MALLOC_DEBUG
+	malloc_printf("Bootstrap allocator: Request to free at %p\n", ptr);
+#endif
+
+	// Do nothing
+	return;
+}
+
+#ifdef MALLOC_STATS
+static void ba_print_stats() {
+	malloc_printf("Bootstrap allocator: %zu bytes requested, "
+		      "%zu allocated\n",
+		      ba_requested_bn, ba_allocated_bn);
+}
+#endif
+
+
+__attribute__((constructor))
+void
+jemalloc_FreeBSD_init(void)
+{
+	if (malloc_init())
+		abort();
+}
+#endif  // #ifdef __FreeBSD__
+
+
 #if !defined(MOZ_MEMORY_WINDOWS)
 static
 #endif
@@ -5016,7 +5247,7 @@ malloc_init_hard(void)
    malloc_zone_t* default_zone;
 #endif

-#ifndef MOZ_MEMORY_WINDOWS
+#if !(defined(MOZ_MEMORY_WINDOWS) || defined(__FreeBSD__))
 	malloc_mutex_lock(&init_lock);
 #endif

@@ -5414,7 +5645,7 @@ MALLOC_OUT:
 	/* Allocate and initialize arenas. */
 	arenas = (arena_t **)base_alloc(sizeof(arena_t *) * narenas);
 	if (arenas == NULL) {
-#ifndef MOZ_MEMORY_WINDOWS
+#if !(defined(MOZ_MEMORY_WINDOWS) || defined(__FreeBSD__))
 		malloc_mutex_unlock(&init_lock);
 #endif
 		return (true);
@@ -5431,7 +5662,7 @@ MALLOC_OUT:
 	 */
 	arenas_extend(0);
 	if (arenas[0] == NULL) {
-#ifndef MOZ_MEMORY_WINDOWS
+#if !(defined(MOZ_MEMORY_WINDOWS) || defined(__FreeBSD__))
 		malloc_mutex_unlock(&init_lock);
 #endif
 		return (true);
@@ -5504,9 +5735,15 @@ MALLOC_OUT:
 	}
 #endif

-#ifndef MOZ_MEMORY_WINDOWS
+#if defined(__FreeBSD__) && defined(MALLOC_STATS)
+	malloc_printf("Bootstrap allocator: malloc_init_hard stats:\n");
+	ba_print_stats();
+#endif
+
+#if !(defined(MOZ_MEMORY_WINDOWS) || defined(__FreeBSD__))
 	malloc_mutex_unlock(&init_lock);
 #endif
+
 	return (false);
 }

@@ -5547,13 +5784,22 @@ malloc_shutdown()
 #define DARWIN_ONLY(A)
 #endif

+#ifdef __FreeBSD__
+#define FREEBSD_ONLY(code) code
+#else
+#define FREEBSD_ONLY(code)
+#endif
+
+
 MOZ_MEMORY_API void *
 malloc_impl(size_t size)
 {
-	void *ret;
-
 	DARWIN_ONLY(return (szone->malloc)(szone, size));

+	FREEBSD_ONLY(BA_DIVERT(return ba_malloc(size)));
+
+	void *ret;
+
 	if (malloc_init()) {
 		ret = NULL;
 		goto RETURN;
@@ -5629,10 +5875,12 @@ MOZ_MEMORY_API
 void *
 MEMALIGN(size_t alignment, size_t size)
 {
-	void *ret;
-
 	DARWIN_ONLY(return (szone->memalign)(szone, alignment, size));

+	FREEBSD_ONLY(BA_DIVERT(return ba_memalign(alignment, size)));
+
+	void *ret;
+
 	assert(((alignment - 1) & alignment) == 0);

 	if (malloc_init()) {
@@ -5727,11 +5975,13 @@ valloc_impl(size_t size)
 MOZ_MEMORY_API void *
 calloc_impl(size_t num, size_t size)
 {
+	DARWIN_ONLY(return (szone->calloc)(szone, num, size));
+
+	FREEBSD_ONLY(BA_DIVERT(return ba_calloc(num, size)));
+
 	void *ret;
 	size_t num_size;

-	DARWIN_ONLY(return (szone->calloc)(szone, num, size));
-
 	if (malloc_init()) {
 		num_size = 0;
 		ret = NULL;
@@ -5846,10 +6096,12 @@ RETURN:
 MOZ_MEMORY_API void
 free_impl(void *ptr)
 {
-	size_t offset;
-
 	DARWIN_ONLY((szone->free)(szone, ptr); return);

+	FREEBSD_ONLY(BA_DIVERT(return ba_free(ptr)));
+
+	size_t offset;
+
 	/*
 	 * A version of idalloc that checks for NULL pointer but only for
 	 * huge allocations assuming that CHUNK_ADDR2OFFSET(NULL) == 0.