Add a new function dllockinit() for registering thread locking

functions to be used by the dynamic linker.  This can be called by
threads packages at start-up time.  I will add the call to libc_r
soon.

Also add a default locking method that is used up until dllockinit()
is called.  The default method works by blocking SIGVTALRM, SIGPROF,
and SIGALRM in critical sections.  It is based on the observation
that most user-space threads packages implement thread preemption
with one of these signals (usually SIGVTALRM).

The dynamic linker has never been reentrant, but it became less
reentrant in revision 1.34 of "src/libexec/rtld-elf/rtld.c".
Starting with that revision, multiple threads each doing lazy
binding could interfere with each other.  The usual symptom was
that a symbol was falsely reported as undefined at start-up time.
It was rare but not unseen.  This commit fixes it.

11 files changed, 509 insertions, 45 deletions

diff --git a/libexec/rtld-elf/Makefile b/libexec/rtld-elf/Makefile
index 4b02d4b..29935143 100644
--- a/libexec/rtld-elf/Makefile
+++ b/libexec/rtld-elf/Makefile
@@ -3,8 +3,8 @@
 #
 MAINTAINER=	jdp
 PROG=		ld-elf.so.1
-SRCS=		rtld_start.S rtld.c map_object.c malloc.c xmalloc.c debug.c \
-		reloc.c
+SRCS=		rtld_start.S rtld.c lockdflt.c map_object.c malloc.c \
+		xmalloc.c debug.c reloc.c
 NOMAN=		true
 CFLAGS+=	-Wall -DFREEBSD_ELF -I${.CURDIR}/${MACHINE_ARCH} -I${.CURDIR}
 LDFLAGS+=	-nostdlib -e .rtld_start
diff --git a/libexec/rtld-elf/alpha/lockdflt.c b/libexec/rtld-elf/alpha/lockdflt.c
new file mode 100644
index 0000000..6f278c3
--- /dev/null
+++ b/libexec/rtld-elf/alpha/lockdflt.c
@@ -0,0 +1,94 @@
+/*-
+ * Copyright 1999 John D. Polstra.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+ * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * $FreeBSD$
+ */
+
+/*
+ * Default thread locking implementation for the dynamic linker.  It
+ * is used until the client registers a different implementation with
+ * dllockinit().  The default implementation does mutual exclusion
+ * by blocking the SIGVTALRM, SIGPROF, and SIGALRM signals.  This is
+ * based on the observation that most userland thread packages use one
+ * of these signals to support preemption.
+ */
+
+#include <dlfcn.h>
+#include <signal.h>
+#include <stdlib.h>
+
+#include "debug.h"
+#include "rtld.h"
+
+typedef struct Struct_LockDflt {
+    sigset_t lock_mask;
+    sigset_t old_mask;
+    int depth;
+} LockDflt;
+
+void
+lockdflt_acquire(void *lock)
+{
+    LockDflt *l = (LockDflt *)lock;
+    sigprocmask(SIG_BLOCK, &l->lock_mask, &l->old_mask);
+    assert(l->depth == 0);
+    l->depth++;
+}
+
+void *
+lockdflt_create(void *context)
+{
+    LockDflt *l;
+
+    l = NEW(LockDflt);
+    l->depth = 0;
+    sigemptyset(&l->lock_mask);
+    sigaddset(&l->lock_mask, SIGVTALRM);
+    sigaddset(&l->lock_mask, SIGPROF);
+    sigaddset(&l->lock_mask, SIGALRM);
+    return l;
+}
+
+void
+lockdflt_destroy(void *lock)
+{
+    LockDflt *l = (LockDflt *)lock;
+    free(l);
+}
+
+void
+lockdflt_release(void *lock)
+{
+    LockDflt *l = (LockDflt *)lock;
+    assert(l->depth == 1);
+    l->depth--;
+    sigprocmask(SIG_SETMASK, &l->old_mask, NULL);
+}
+
+void
+lockdflt_init(void)
+{
+    dllockinit(NULL, lockdflt_create, lockdflt_acquire, lockdflt_acquire,
+      lockdflt_release, lockdflt_destroy, NULL);
+}
diff --git a/libexec/rtld-elf/alpha/reloc.c b/libexec/rtld-elf/alpha/reloc.c
index 1b55d06..114f3ae 100644
--- a/libexec/rtld-elf/alpha/reloc.c
+++ b/libexec/rtld-elf/alpha/reloc.c
@@ -47,15 +47,6 @@
 #include "debug.h"
 #include "rtld.h"
 
-/*
- * Debugging support.
- */
-
-#define assert(cond)	((cond) ? (void) 0 :\
-    (msg("oops: " __XSTRING(__LINE__) "\n"), abort()))
-#define msg(s)		(write(1, s, strlen(s)))
-#define trace()		msg("trace: " __XSTRING(__LINE__) "\n");
-
 extern Elf_Dyn _DYNAMIC;
 
 /* Relocate a non-PLT object with addend. */
diff --git a/libexec/rtld-elf/amd64/lockdflt.c b/libexec/rtld-elf/amd64/lockdflt.c
new file mode 100644
index 0000000..6f278c3
--- /dev/null
+++ b/libexec/rtld-elf/amd64/lockdflt.c
@@ -0,0 +1,94 @@
+/*-
+ * Copyright 1999 John D. Polstra.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+ * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * $FreeBSD$
+ */
+
+/*
+ * Default thread locking implementation for the dynamic linker.  It
+ * is used until the client registers a different implementation with
+ * dllockinit().  The default implementation does mutual exclusion
+ * by blocking the SIGVTALRM, SIGPROF, and SIGALRM signals.  This is
+ * based on the observation that most userland thread packages use one
+ * of these signals to support preemption.
+ */
+
+#include <dlfcn.h>
+#include <signal.h>
+#include <stdlib.h>
+
+#include "debug.h"
+#include "rtld.h"
+
+typedef struct Struct_LockDflt {
+    sigset_t lock_mask;
+    sigset_t old_mask;
+    int depth;
+} LockDflt;
+
+void
+lockdflt_acquire(void *lock)
+{
+    LockDflt *l = (LockDflt *)lock;
+    sigprocmask(SIG_BLOCK, &l->lock_mask, &l->old_mask);
+    assert(l->depth == 0);
+    l->depth++;
+}
+
+void *
+lockdflt_create(void *context)
+{
+    LockDflt *l;
+
+    l = NEW(LockDflt);
+    l->depth = 0;
+    sigemptyset(&l->lock_mask);
+    sigaddset(&l->lock_mask, SIGVTALRM);
+    sigaddset(&l->lock_mask, SIGPROF);
+    sigaddset(&l->lock_mask, SIGALRM);
+    return l;
+}
+
+void
+lockdflt_destroy(void *lock)
+{
+    LockDflt *l = (LockDflt *)lock;
+    free(l);
+}
+
+void
+lockdflt_release(void *lock)
+{
+    LockDflt *l = (LockDflt *)lock;
+    assert(l->depth == 1);
+    l->depth--;
+    sigprocmask(SIG_SETMASK, &l->old_mask, NULL);
+}
+
+void
+lockdflt_init(void)
+{
+    dllockinit(NULL, lockdflt_create, lockdflt_acquire, lockdflt_acquire,
+      lockdflt_release, lockdflt_destroy, NULL);
+}
diff --git a/libexec/rtld-elf/amd64/reloc.c b/libexec/rtld-elf/amd64/reloc.c
index b4a2a75..217d88e 100644
--- a/libexec/rtld-elf/amd64/reloc.c
+++ b/libexec/rtld-elf/amd64/reloc.c
@@ -48,15 +48,6 @@
 #include "rtld.h"
 
 /*
- * Debugging support.
- */
-
-#define assert(cond)	((cond) ? (void) 0 :\
-    (msg("oops: " __XSTRING(__LINE__) "\n"), abort()))
-#define msg(s)		(write(1, s, strlen(s)))
-#define trace()		msg("trace: " __XSTRING(__LINE__) "\n");
-
-/*
  * Process the special R_386_COPY relocations in the main program.  These
  * copy data from a shared object into a region in the main program's BSS
  * segment.
diff --git a/libexec/rtld-elf/debug.h b/libexec/rtld-elf/debug.h
index b356338..2600651 100644
--- a/libexec/rtld-elf/debug.h
+++ b/libexec/rtld-elf/debug.h
@@ -38,6 +38,9 @@
 
 #include <sys/cdefs.h>
 
+#include <string.h>
+#include <unistd.h>
+
 extern void debug_printf(const char *, ...) __printflike(1, 2);
 extern int debug;
 
@@ -47,4 +50,10 @@ extern int debug;
 #define dbg(format, args...)	((void) 0)
 #endif
 
+#define assert(cond)	((cond) ? (void) 0 :		\
+    (msg("ld-elf.so.1: assert failed: " __FILE__ ":"	\
+      __XSTRING(__LINE__) "\n"), abort()))
+#define msg(s)		write(1, s, strlen(s))
+#define trace()		msg("ld-elf.so.1: " __XSTRING(__LINE__) "\n")
+
 #endif /* DEBUG_H */
diff --git a/libexec/rtld-elf/i386/lockdflt.c b/libexec/rtld-elf/i386/lockdflt.c
new file mode 100644
index 0000000..6f278c3
--- /dev/null
+++ b/libexec/rtld-elf/i386/lockdflt.c
@@ -0,0 +1,94 @@
+/*-
+ * Copyright 1999 John D. Polstra.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+ * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * $FreeBSD$
+ */
+
+/*
+ * Default thread locking implementation for the dynamic linker.  It
+ * is used until the client registers a different implementation with
+ * dllockinit().  The default implementation does mutual exclusion
+ * by blocking the SIGVTALRM, SIGPROF, and SIGALRM signals.  This is
+ * based on the observation that most userland thread packages use one
+ * of these signals to support preemption.
+ */
+
+#include <dlfcn.h>
+#include <signal.h>
+#include <stdlib.h>
+
+#include "debug.h"
+#include "rtld.h"
+
+typedef struct Struct_LockDflt {
+    sigset_t lock_mask;
+    sigset_t old_mask;
+    int depth;
+} LockDflt;
+
+void
+lockdflt_acquire(void *lock)
+{
+    LockDflt *l = (LockDflt *)lock;
+    sigprocmask(SIG_BLOCK, &l->lock_mask, &l->old_mask);
+    assert(l->depth == 0);
+    l->depth++;
+}
+
+void *
+lockdflt_create(void *context)
+{
+    LockDflt *l;
+
+    l = NEW(LockDflt);
+    l->depth = 0;
+    sigemptyset(&l->lock_mask);
+    sigaddset(&l->lock_mask, SIGVTALRM);
+    sigaddset(&l->lock_mask, SIGPROF);
+    sigaddset(&l->lock_mask, SIGALRM);
+    return l;
+}
+
+void
+lockdflt_destroy(void *lock)
+{
+    LockDflt *l = (LockDflt *)lock;
+    free(l);
+}
+
+void
+lockdflt_release(void *lock)
+{
+    LockDflt *l = (LockDflt *)lock;
+    assert(l->depth == 1);
+    l->depth--;
+    sigprocmask(SIG_SETMASK, &l->old_mask, NULL);
+}
+
+void
+lockdflt_init(void)
+{
+    dllockinit(NULL, lockdflt_create, lockdflt_acquire, lockdflt_acquire,
+      lockdflt_release, lockdflt_destroy, NULL);
+}
diff --git a/libexec/rtld-elf/i386/reloc.c b/libexec/rtld-elf/i386/reloc.c
index b4a2a75..217d88e 100644
--- a/libexec/rtld-elf/i386/reloc.c
+++ b/libexec/rtld-elf/i386/reloc.c
@@ -48,15 +48,6 @@
 #include "rtld.h"
 
 /*
- * Debugging support.
- */
-
-#define assert(cond)	((cond) ? (void) 0 :\
-    (msg("oops: " __XSTRING(__LINE__) "\n"), abort()))
-#define msg(s)		(write(1, s, strlen(s)))
-#define trace()		msg("trace: " __XSTRING(__LINE__) "\n");
-
-/*
  * Process the special R_386_COPY relocations in the main program.  These
  * copy data from a shared object into a region in the main program's BSS
  * segment.
diff --git a/libexec/rtld-elf/lockdflt.c b/libexec/rtld-elf/lockdflt.c
new file mode 100644
index 0000000..6f278c3
--- /dev/null
+++ b/libexec/rtld-elf/lockdflt.c
@@ -0,0 +1,94 @@
+/*-
+ * Copyright 1999 John D. Polstra.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+ * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * $FreeBSD$
+ */
+
+/*
+ * Default thread locking implementation for the dynamic linker.  It
+ * is used until the client registers a different implementation with
+ * dllockinit().  The default implementation does mutual exclusion
+ * by blocking the SIGVTALRM, SIGPROF, and SIGALRM signals.  This is
+ * based on the observation that most userland thread packages use one
+ * of these signals to support preemption.
+ */
+
+#include <dlfcn.h>
+#include <signal.h>
+#include <stdlib.h>
+
+#include "debug.h"
+#include "rtld.h"
+
+typedef struct Struct_LockDflt {
+    sigset_t lock_mask;
+    sigset_t old_mask;
+    int depth;
+} LockDflt;
+
+void
+lockdflt_acquire(void *lock)
+{
+    LockDflt *l = (LockDflt *)lock;
+    sigprocmask(SIG_BLOCK, &l->lock_mask, &l->old_mask);
+    assert(l->depth == 0);
+    l->depth++;
+}
+
+void *
+lockdflt_create(void *context)
+{
+    LockDflt *l;
+
+    l = NEW(LockDflt);
+    l->depth = 0;
+    sigemptyset(&l->lock_mask);
+    sigaddset(&l->lock_mask, SIGVTALRM);
+    sigaddset(&l->lock_mask, SIGPROF);
+    sigaddset(&l->lock_mask, SIGALRM);
+    return l;
+}
+
+void
+lockdflt_destroy(void *lock)
+{
+    LockDflt *l = (LockDflt *)lock;
+    free(l);
+}
+
+void
+lockdflt_release(void *lock)
+{
+    LockDflt *l = (LockDflt *)lock;
+    assert(l->depth == 1);
+    l->depth--;
+    sigprocmask(SIG_SETMASK, &l->old_mask, NULL);
+}
+
+void
+lockdflt_init(void)
+{
+    dllockinit(NULL, lockdflt_create, lockdflt_acquire, lockdflt_acquire,
+      lockdflt_release, lockdflt_destroy, NULL);
+}
diff --git a/libexec/rtld-elf/rtld.c b/libexec/rtld-elf/rtld.c
index 8ba21b4..5bcd390 100644
--- a/libexec/rtld-elf/rtld.c
+++ b/libexec/rtld-elf/rtld.c
@@ -52,21 +52,23 @@
 #include "debug.h"
 #include "rtld.h"
 
-/*
- * Debugging support.
- */
-
-#define assert(cond)	((cond) ? (void) 0 :\
-    (msg("oops: " __XSTRING(__LINE__) "\n"), abort()))
-#define msg(s)		(write(1, s, strlen(s)))
-#define trace()		msg("trace: " __XSTRING(__LINE__) "\n");
-
 #define END_SYM		"_end"
 #define PATH_RTLD	"/usr/libexec/ld-elf.so.1"
 
 /* Types. */
 typedef void (*func_ptr_type)();
 
+typedef struct Struct_LockInfo {
+    void *context;		/* Client context for creating locks */
+    void *thelock;		/* The one big lock */
+    /* Methods */
+    void (*rlock_acquire)(void *lock);
+    void (*wlock_acquire)(void *lock);
+    void (*lock_release)(void *lock);
+    void (*lock_destroy)(void *lock);
+    void (*context_destroy)(void *context);
+} LockInfo;
+
 /*
  * Function declarations.
  */
@@ -88,6 +90,7 @@ static void linkmap_delete(Obj_Entry *);
 static int load_needed_objects(Obj_Entry *);
 static int load_preload_objects(void);
 static Obj_Entry *load_object(char *);
+static void lock_nop(void *);
 static Obj_Entry *obj_from_addr(const void *);
 static void objlist_add(Objlist *, Obj_Entry *);
 static Objlist_Entry *objlist_find(Objlist *, const Obj_Entry *);
@@ -128,6 +131,8 @@ static Objlist list_global =	/* Objects dlopened with RTLD_GLOBAL */
 static Objlist list_main =	/* Objects loaded at program startup */
   STAILQ_HEAD_INITIALIZER(list_main);
 
+static LockInfo lockinfo;
+
 static Elf_Sym sym_zero;	/* For resolving undefined weak refs. */
 
 #define GDB_STATE(s)	r_debug.r_state = s; r_debug_state();
@@ -147,6 +152,7 @@ static func_ptr_type exports[] = {
     (func_ptr_type) &dlopen,
     (func_ptr_type) &dlsym,
     (func_ptr_type) &dladdr,
+    (func_ptr_type) &dllockinit,
     NULL
 };
 
@@ -157,6 +163,24 @@ static func_ptr_type exports[] = {
 char *__progname;
 char **environ;
 
+static __inline void
+rlock_acquire(void)
+{
+    lockinfo.rlock_acquire(lockinfo.thelock);
+}
+
+static __inline void
+wlock_acquire(void)
+{
+    lockinfo.wlock_acquire(lockinfo.thelock);
+}
+
+static __inline void
+lock_release(void)
+{
+    lockinfo.lock_release(lockinfo.thelock);
+}
+
 /*
  * Main entry point for dynamic linking.  The first argument is the
  * stack pointer.  The stack is expected to be laid out as described
@@ -322,6 +346,9 @@ _rtld(Elf_Addr *sp, func_ptr_type *exit_proc, Obj_Entry **objp)
     set_program_var("__progname", argv[0] != NULL ? basename(argv[0]) : "");
     set_program_var("environ", env);
 
+    dbg("initializing default locks");
+    dllockinit(NULL, NULL, NULL, NULL, NULL, NULL, NULL);
+
     r_debug_state();		/* say hello to gdb! */
 
     dbg("calling _init functions");
@@ -344,6 +371,7 @@ _rtld_bind(Obj_Entry *obj, Elf_Word reloff)
     Elf_Addr *where;
     Elf_Addr target;
 
+    wlock_acquire();
     if (obj->pltrel)
 	rel = (const Elf_Rel *) ((caddr_t) obj->pltrel + reloff);
     else
@@ -361,6 +389,7 @@ _rtld_bind(Obj_Entry *obj, Elf_Word reloff)
       (void *)target, basename(defobj->path));
 
     reloc_jmpslot(where, target);
+    lock_release();
     return target;
 }
 
@@ -1058,6 +1087,11 @@ load_object(char *path)
     return obj;
 }
 
+static void
+lock_nop(void *lock)
+{
+}
+
 static Obj_Entry *
 obj_from_addr(const void *addr)
 {
@@ -1223,16 +1257,21 @@ search_library_path(const char *name, const char *path)
 int
 dlclose(void *handle)
 {
-    Obj_Entry *root = dlcheck(handle);
+    Obj_Entry *root;
 
-    if (root == NULL)
+    wlock_acquire();
+    root = dlcheck(handle);
+    if (root == NULL) {
+	lock_release();
 	return -1;
+    }
 
     GDB_STATE(RT_DELETE);
     unload_object(root, true);
     root->dl_refcount--;
     GDB_STATE(RT_CONSISTENT);
 
+    lock_release();
     return 0;
 }
 
@@ -1244,14 +1283,67 @@ dlerror(void)
     return msg;
 }
 
+void
+dllockinit(void *context,
+	   void *(*lock_create)(void *context),
+           void (*rlock_acquire)(void *lock),
+           void (*wlock_acquire)(void *lock),
+           void (*lock_release)(void *lock),
+           void (*lock_destroy)(void *lock),
+	   void (*context_destroy)(void *context))
+{
+    /* NULL arguments mean reset to the built-in locks. */
+    if (lock_create == NULL) {
+	context = NULL;
+	lock_create = lockdflt_create;
+	rlock_acquire = wlock_acquire = lockdflt_acquire;
+	lock_release = lockdflt_release;
+	lock_destroy = lockdflt_destroy;
+	context_destroy = NULL;
+    }
+
+    /* Temporarily set locking methods to no-ops. */
+    lockinfo.rlock_acquire = lock_nop;
+    lockinfo.wlock_acquire = lock_nop;
+    lockinfo.lock_release = lock_nop;
+
+    /* Release any existing locks and context. */
+    if (lockinfo.lock_destroy != NULL)
+	lockinfo.lock_destroy(lockinfo.thelock);
+    if (lockinfo.context_destroy != NULL)
+	lockinfo.context_destroy(lockinfo.context);
+
+    /*
+     * Allocate the locks we will need and call all the new locking
+     * methods, to accomplish any needed lazy binding for the methods
+     * themselves.
+     */
+    lockinfo.thelock = lock_create(lockinfo.context);
+    rlock_acquire(lockinfo.thelock);
+    lock_release(lockinfo.thelock);
+    wlock_acquire(lockinfo.thelock);
+    lock_release(lockinfo.thelock);
+
+    /* Record the new method information. */
+    lockinfo.context = context;
+    lockinfo.rlock_acquire = rlock_acquire;
+    lockinfo.wlock_acquire = wlock_acquire;
+    lockinfo.lock_release = lock_release;
+    lockinfo.lock_destroy = lock_destroy;
+    lockinfo.context_destroy = context_destroy;
+}
+
 void *
 dlopen(const char *name, int mode)
 {
-    Obj_Entry **old_obj_tail = obj_tail;
-    Obj_Entry *obj = NULL;
+    Obj_Entry **old_obj_tail;
+    Obj_Entry *obj;
 
+    wlock_acquire();
     GDB_STATE(RT_ADD);
 
+    old_obj_tail = obj_tail;
+    obj = NULL;
     if (name == NULL) {
 	obj = obj_main;
 	obj->refcount++;
@@ -1280,7 +1372,7 @@ dlopen(const char *name, int mode)
     }
 
     GDB_STATE(RT_CONSISTENT);
-
+    lock_release();
     return obj;
 }
 
@@ -1296,12 +1388,14 @@ dlsym(void *handle, const char *name)
     def = NULL;
     defobj = NULL;
 
+    wlock_acquire();
     if (handle == NULL || handle == RTLD_NEXT) {
 	void *retaddr;
 
 	retaddr = __builtin_return_address(0);	/* __GNUC__ only */
 	if ((obj = obj_from_addr(retaddr)) == NULL) {
 	    _rtld_error("Cannot determine caller's shared object");
+	    lock_release();
 	    return NULL;
 	}
 	if (handle == NULL) {	/* Just the caller's shared object. */
@@ -1316,8 +1410,10 @@ dlsym(void *handle, const char *name)
 	    }
 	}
     } else {
-	if ((obj = dlcheck(handle)) == NULL)
+	if ((obj = dlcheck(handle)) == NULL) {
+	    lock_release();
 	    return NULL;
+	}
 
 	if (obj->mainprog) {
 	    /* Search main program and all libraries loaded by it. */
@@ -1333,10 +1429,13 @@ dlsym(void *handle, const char *name)
 	}
     }
 
-    if (def != NULL)
+    if (def != NULL) {
+	lock_release();
 	return defobj->relocbase + def->st_value;
+    }
 
     _rtld_error("Undefined symbol \"%s\"", name);
+    lock_release();
     return NULL;
 }
 
@@ -1348,9 +1447,11 @@ dladdr(const void *addr, Dl_info *info)
     void *symbol_addr;
     unsigned long symoffset;
     
+    wlock_acquire();
     obj = obj_from_addr(addr);
     if (obj == NULL) {
         _rtld_error("No shared object contains address");
+	lock_release();
         return 0;
     }
     info->dli_fname = obj->path;
@@ -1389,6 +1490,7 @@ dladdr(const void *addr, Dl_info *info)
         if (info->dli_saddr == addr)
             break;
     }
+    lock_release();
     return 1;
 }
 
diff --git a/libexec/rtld-elf/rtld.h b/libexec/rtld-elf/rtld.h
index c63e7d0..b8fcf2c 100644
--- a/libexec/rtld-elf/rtld.h
+++ b/libexec/rtld-elf/rtld.h
@@ -157,6 +157,10 @@ unsigned long elf_hash(const char *);
 const Elf_Sym *find_symdef(unsigned long, Obj_Entry *, const Obj_Entry **,
   bool);
 void init_pltgot(Obj_Entry *);
+void lockdflt_acquire(void *);
+void *lockdflt_create(void *);
+void lockdflt_destroy(void *);
+void lockdflt_release(void *);
 void obj_free(Obj_Entry *);
 Obj_Entry *obj_new(void);
 int reloc_non_plt(Obj_Entry *, Obj_Entry *);