0.9.4.1: thread allocation
[sbcl.git] / src / runtime / thread.c
index 4b39120..2324ca7 100644 (file)
@@ -10,7 +10,7 @@
 
 #include "sbcl.h"
 #include "runtime.h"
-#include "validate.h"          /* for CONTROL_STACK_SIZE etc */
+#include "validate.h"           /* for CONTROL_STACK_SIZE etc */
 #include "alloc.h"
 #include "thread.h"
 #include "arch.h"
 
 #define ALIEN_STACK_SIZE (1*1024*1024) /* 1Mb size chosen at random */
 
-int dynamic_values_bytes=4096*sizeof(lispobj); /* same for all threads */
-struct thread *all_threads;
-volatile lispobj all_threads_lock;
+int dynamic_values_bytes=4096*sizeof(lispobj);  /* same for all threads */
+struct thread * volatile all_threads;
 extern struct interrupt_data * global_interrupt_data;
 extern int linux_no_threads_p;
 
+#ifdef LISP_FEATURE_SB_THREAD
+
+pthread_mutex_t all_threads_lock = PTHREAD_MUTEX_INITIALIZER;
+
 /* When trying to get all_threads_lock one should make sure that
  * sig_stop_for_gc is not blocked. Else there would be a possible
  * deadlock: gc locks it, other thread blocks signals, gc sends stop
@@ -42,47 +45,45 @@ void check_sig_stop_for_gc_can_arrive_or_lose()
     sigemptyset(&empty);
     thread_sigmask(SIG_BLOCK, &empty, &current);
     if (sigismember(&current,SIG_STOP_FOR_GC))
-        lose("SIG_STOP_FOR_GC is blocked\n");
-    if (SymbolValue(INTERRUPTS_ENABLED,arch_os_get_current_thread()) == NIL)
-        lose("interrupts disabled\n");
+        lose("SIG_STOP_FOR_GC cannot arrive: it is blocked\n");
+    if (SymbolValue(GC_INHIBIT,arch_os_get_current_thread()) != NIL)
+        lose("SIG_STOP_FOR_GC cannot arrive: gc is inhibited\n");
     if (arch_pseudo_atomic_atomic(NULL))
-        lose("n pseudo atomic\n");
+        lose("SIG_STOP_FOR_GC cannot arrive: in pseudo atomic\n");
 }
 
-#ifdef QSHOW_SIGNALS
-#define FSHOW_SIGNAL FSHOW
-#else
-#define FSHOW_SIGNAL(args)
-#endif
-
 #define GET_ALL_THREADS_LOCK(name) \
     { \
         sigset_t _newset,_oldset; \
         sigemptyset(&_newset); \
-        sigaddset_blockable(&_newset); \
-        sigdelset(&_newset,SIG_STOP_FOR_GC); \
+        sigaddset_deferrable(&_newset); \
         thread_sigmask(SIG_BLOCK, &_newset, &_oldset); \
         check_sig_stop_for_gc_can_arrive_or_lose(); \
-        FSHOW_SIGNAL((stderr,"/%s:waiting on lock=%ld, thread=%ld\n",name, \
+        FSHOW_SIGNAL((stderr,"/%s:waiting on lock=%ld, thread=%lu\n",name, \
                all_threads_lock,arch_os_get_current_thread()->os_thread)); \
-        get_spinlock(&all_threads_lock,(long)arch_os_get_current_thread()); \
-        FSHOW_SIGNAL((stderr,"/%s:got lock, thread=%ld\n", \
+        pthread_mutex_lock(&all_threads_lock); \
+        FSHOW_SIGNAL((stderr,"/%s:got lock, thread=%lu\n", \
                name,arch_os_get_current_thread()->os_thread));
 
 #define RELEASE_ALL_THREADS_LOCK(name) \
         FSHOW_SIGNAL((stderr,"/%s:released lock\n",name)); \
-        release_spinlock(&all_threads_lock); \
+        pthread_mutex_unlock(&all_threads_lock); \
         thread_sigmask(SIG_SETMASK,&_oldset,0); \
     }
+#endif
 
-int
+
+#if defined(LISP_FEATURE_X86) || defined(LISP_FEATURE_X86_64)
+extern lispobj call_into_lisp_first_time(lispobj fun, lispobj *args, int nargs);
+#endif
+
+static int
 initial_thread_trampoline(struct thread *th)
 {
     lispobj function;
 #if defined(LISP_FEATURE_X86) || defined(LISP_FEATURE_X86_64)
     lispobj *args = NULL;
 #endif
-
     function = th->unbound_marker;
     th->unbound_marker = UNBOUND_MARKER_WIDETAG;
     if(arch_os_thread_init(th)==0) return 1;
@@ -107,71 +108,90 @@ int
 new_thread_trampoline(struct thread *th)
 {
     lispobj function;
+    int result;
     function = th->unbound_marker;
     th->unbound_marker = UNBOUND_MARKER_WIDETAG;
-    if(arch_os_thread_init(th)==0) return 1;
+    if(arch_os_thread_init(th)==0) {
+        /* FIXME: handle error */
+        lose("arch_os_thread_init failed\n");
+    }
 
     /* wait here until our thread is linked into all_threads: see below */
-    while(th->os_thread<1) sched_yield();
+    {
+        volatile os_thread_t *tid=&th->os_thread;
+        while(*tid<1) sched_yield();
+    }
 
     th->state=STATE_RUNNING;
-    return funcall0(function);
+    result = funcall0(function);
+    th->state=STATE_DEAD;
+    return result;
 }
 #endif /* LISP_FEATURE_SB_THREAD */
 
+#define THREAD_STRUCT_SIZE (THREAD_CONTROL_STACK_SIZE + BINDING_STACK_SIZE + \
+                            ALIEN_STACK_SIZE + dynamic_values_bytes + \
+                            32 * SIGSTKSZ)
+
+static void
+free_thread_struct(struct thread *th)
+{
+    if (th->interrupt_data)
+        os_invalidate((os_vm_address_t) th->interrupt_data,
+                      (sizeof (struct interrupt_data)));
+    os_invalidate((os_vm_address_t) th->control_stack_start,
+                  THREAD_STRUCT_SIZE);
+}
+
 /* this is called from any other thread to create the new one, and
  * initialize all parts of it that can be initialized from another
  * thread
  */
 
-struct thread * create_thread_struct(lispobj initial_function) {
+static struct thread *
+create_thread_struct(lispobj initial_function) {
     union per_thread_data *per_thread;
-    struct thread *th=0;       /*  subdue gcc */
+    struct thread *th=0;        /*  subdue gcc */
     void *spaces=0;
 
     /* may as well allocate all the spaces at once: it saves us from
      * having to decide what to do if only some of the allocations
      * succeed */
-    spaces=os_validate(0,
-                      THREAD_CONTROL_STACK_SIZE+
-                      BINDING_STACK_SIZE+
-                      ALIEN_STACK_SIZE+
-                      dynamic_values_bytes+
-                      32*SIGSTKSZ);
+    spaces=os_validate(0, THREAD_STRUCT_SIZE);
     if(!spaces)
-        return NULL;
+         return NULL;
     per_thread=(union per_thread_data *)
-       (spaces+
-        THREAD_CONTROL_STACK_SIZE+
-        BINDING_STACK_SIZE+
-        ALIEN_STACK_SIZE);
+        (spaces+
+         THREAD_CONTROL_STACK_SIZE+
+         BINDING_STACK_SIZE+
+         ALIEN_STACK_SIZE);
 
     if(all_threads) {
-       memcpy(per_thread,arch_os_get_current_thread(),
-              dynamic_values_bytes);
+        memcpy(per_thread,arch_os_get_current_thread(),
+               dynamic_values_bytes);
     } else {
 #ifdef LISP_FEATURE_SB_THREAD
-       int i;
-       for(i=0;i<(dynamic_values_bytes/sizeof(lispobj));i++)
-           per_thread->dynamic_values[i]=UNBOUND_MARKER_WIDETAG;
-       if(SymbolValue(FREE_TLS_INDEX,0)==UNBOUND_MARKER_WIDETAG)
-           SetSymbolValue
-               (FREE_TLS_INDEX,
-                make_fixnum(MAX_INTERRUPTS+
-                            sizeof(struct thread)/sizeof(lispobj)),
-                0);
+        int i;
+        for(i=0;i<(dynamic_values_bytes/sizeof(lispobj));i++)
+            per_thread->dynamic_values[i]=UNBOUND_MARKER_WIDETAG;
+        if(SymbolValue(FREE_TLS_INDEX,0)==UNBOUND_MARKER_WIDETAG)
+            SetSymbolValue
+                (FREE_TLS_INDEX,
+                 make_fixnum(MAX_INTERRUPTS+
+                             sizeof(struct thread)/sizeof(lispobj)),
+                 0);
 #define STATIC_TLS_INIT(sym,field) \
   ((struct symbol *)(sym-OTHER_POINTER_LOWTAG))->tls_index= \
   make_fixnum(THREAD_SLOT_OFFSET_WORDS(field))
 
-       STATIC_TLS_INIT(BINDING_STACK_START,binding_stack_start);
-       STATIC_TLS_INIT(BINDING_STACK_POINTER,binding_stack_pointer);
-       STATIC_TLS_INIT(CONTROL_STACK_START,control_stack_start);
-       STATIC_TLS_INIT(CONTROL_STACK_END,control_stack_end);
-       STATIC_TLS_INIT(ALIEN_STACK,alien_stack_pointer);
+        STATIC_TLS_INIT(BINDING_STACK_START,binding_stack_start);
+        STATIC_TLS_INIT(BINDING_STACK_POINTER,binding_stack_pointer);
+        STATIC_TLS_INIT(CONTROL_STACK_START,control_stack_start);
+        STATIC_TLS_INIT(CONTROL_STACK_END,control_stack_end);
+        STATIC_TLS_INIT(ALIEN_STACK,alien_stack_pointer);
 #if defined(LISP_FEATURE_X86) || defined (LISP_FEATURE_X86_64)
-       STATIC_TLS_INIT(PSEUDO_ATOMIC_ATOMIC,pseudo_atomic_atomic);
-       STATIC_TLS_INIT(PSEUDO_ATOMIC_INTERRUPTED,pseudo_atomic_interrupted);
+        STATIC_TLS_INIT(PSEUDO_ATOMIC_ATOMIC,pseudo_atomic_atomic);
+        STATIC_TLS_INIT(PSEUDO_ATOMIC_INTERRUPTED,pseudo_atomic_interrupted);
 #endif
 #undef STATIC_TLS_INIT
 #endif
@@ -180,10 +200,10 @@ struct thread * create_thread_struct(lispobj initial_function) {
     th=&per_thread->thread;
     th->control_stack_start = spaces;
     th->binding_stack_start=
-       (lispobj*)((void*)th->control_stack_start+THREAD_CONTROL_STACK_SIZE);
+        (lispobj*)((void*)th->control_stack_start+THREAD_CONTROL_STACK_SIZE);
     th->control_stack_end = th->binding_stack_start;
     th->alien_stack_start=
-       (lispobj*)((void*)th->binding_stack_start+BINDING_STACK_SIZE);
+        (lispobj*)((void*)th->binding_stack_start+BINDING_STACK_SIZE);
     th->binding_stack_pointer=th->binding_stack_start;
     th->this=th;
     th->os_thread=0;
@@ -192,7 +212,7 @@ struct thread * create_thread_struct(lispobj initial_function) {
     th->state=STATE_STARTING;
 #ifdef LISP_FEATURE_STACK_GROWS_DOWNWARD_NOT_UPWARD
     th->alien_stack_pointer=((void *)th->alien_stack_start
-                            + ALIEN_STACK_SIZE-N_WORD_BYTES);
+                             + ALIEN_STACK_SIZE-N_WORD_BYTES);
 #else
     th->alien_stack_pointer=((void *)th->alien_stack_start);
 #endif
@@ -229,22 +249,31 @@ struct thread * create_thread_struct(lispobj initial_function) {
     bind_variable(FREE_INTERRUPT_CONTEXT_INDEX,make_fixnum(0),th);
     bind_variable(INTERRUPT_PENDING, NIL,th);
     bind_variable(INTERRUPTS_ENABLED,T,th);
+    bind_variable(GC_PENDING,NIL,th);
+#ifdef LISP_FEATURE_SB_THREAD
+    bind_variable(STOP_FOR_GC_PENDING,NIL,th);
+#endif
 
     th->interrupt_data = (struct interrupt_data *)
         os_validate(0,(sizeof (struct interrupt_data)));
+    if (!th->interrupt_data) {
+        free_thread_struct(th);
+        return 0;
+    }
     if(all_threads)
-       memcpy(th->interrupt_data,
-              arch_os_get_current_thread()->interrupt_data,
-              sizeof (struct interrupt_data));
+        memcpy(th->interrupt_data,
+               arch_os_get_current_thread()->interrupt_data,
+               sizeof (struct interrupt_data));
     else
-       memcpy(th->interrupt_data,global_interrupt_data,
-              sizeof (struct interrupt_data));
+        memcpy(th->interrupt_data,global_interrupt_data,
+               sizeof (struct interrupt_data));
 
     th->unbound_marker=initial_function;
     return th;
 }
 
-void link_thread(struct thread *th,os_thread_t kid_tid)
+static void
+link_thread(struct thread *th,os_thread_t kid_tid)
 {
     if (all_threads) all_threads->prev=th;
     th->next=all_threads;
@@ -257,19 +286,25 @@ void link_thread(struct thread *th,os_thread_t kid_tid)
     protect_control_stack_guard_page(th,1);
     /* child will not start until this is set */
     th->os_thread=kid_tid;
+    FSHOW((stderr,"/created thread %lu\n",kid_tid));
 }
 
 void create_initial_thread(lispobj initial_function) {
     struct thread *th=create_thread_struct(initial_function);
     os_thread_t kid_tid=thread_self();
     if(th && kid_tid>0) {
-       link_thread(th,kid_tid);
-       initial_thread_trampoline(all_threads); /* no return */
+        link_thread(th,kid_tid);
+        initial_thread_trampoline(all_threads); /* no return */
     } else lose("can't create initial thread");
 }
 
 #ifdef LISP_FEATURE_SB_THREAD
 
+#ifndef __USE_XOPEN2K
+extern int pthread_attr_setstack (pthread_attr_t *__attr, void *__stackaddr,
+                                  size_t __stacksize);
+#endif
+
 boolean create_os_thread(struct thread *th,os_thread_t *kid_tid)
 {
     /* The new thread inherits the restrictive signal mask set here,
@@ -278,9 +313,12 @@ boolean create_os_thread(struct thread *th,os_thread_t *kid_tid)
     sigset_t newset,oldset;
     boolean r=1;
     sigemptyset(&newset);
-    sigaddset_blockable(&newset);
+    /* Blocking deferrable signals is enough, since gc_stop_the_world
+     * waits until the child leaves STATE_STARTING. And why not let gc
+     * proceed as soon as possible? */
+    sigaddset_deferrable(&newset);
     thread_sigmask(SIG_BLOCK, &newset, &oldset);
-    
+
     if((pthread_attr_init(&attr)) ||
        (pthread_attr_setstack(&attr,th->control_stack_start,
                               THREAD_CONTROL_STACK_SIZE-16)) ||
@@ -308,13 +346,9 @@ struct thread *create_thread(lispobj initial_function) {
 
     success=create_os_thread(th,&kid_tid);
     if (success)
-       link_thread(th,kid_tid);
+        link_thread(th,kid_tid);
     else
-       os_invalidate((os_vm_address_t) th->control_stack_start,
-                     ((sizeof (lispobj))
-                      * (th->control_stack_end-th->control_stack_start)) +
-                     BINDING_STACK_SIZE+ALIEN_STACK_SIZE+dynamic_values_bytes+
-                     32*SIGSTKSZ);
+        free_thread_struct(th);
 
     RELEASE_ALL_THREADS_LOCK("create_thread")
 
@@ -323,17 +357,12 @@ struct thread *create_thread(lispobj initial_function) {
     else
         return 0;
 }
-#endif
-
-#if defined LISP_FEATURE_SB_THREAD
-/* This is not needed unless #+SB-THREAD, as there's a trivial null
- * unithread definition. */
 
 /* called from lisp from the thread object finalizer */
 void reap_dead_thread(struct thread *th)
 {
     if(th->state!=STATE_DEAD)
-        lose("thread %lx is not joinable, state=%d\n",th,th->state);
+        lose("thread %p is not joinable, state=%d\n",th,th->state);
 #ifdef LISP_FEATURE_GENCGC
     {
         sigset_t newset,oldset;
@@ -341,12 +370,11 @@ void reap_dead_thread(struct thread *th)
         sigaddset_blockable(&newset);
         thread_sigmask(SIG_BLOCK, &newset, &oldset);
         gc_alloc_update_page_tables(0, &th->alloc_region);
-        release_spinlock(&all_threads_lock);
         thread_sigmask(SIG_SETMASK,&oldset,0);
     }
 #endif
     GET_ALL_THREADS_LOCK("reap_dead_thread")
-    FSHOW((stderr,"/reap_dead_thread: reaping %ld\n",th->os_thread));
+    FSHOW((stderr,"/reap_dead_thread: reaping %lu\n",th->os_thread));
     if(th->prev)
         th->prev->next=th->next;
     else all_threads=th->next;
@@ -355,53 +383,81 @@ void reap_dead_thread(struct thread *th)
     RELEASE_ALL_THREADS_LOCK("reap_dead_thread")
     if(th->tls_cookie>=0) arch_os_thread_cleanup(th);
     gc_assert(pthread_join(th->os_thread,NULL)==0);
-    os_invalidate((os_vm_address_t) th->control_stack_start,
-                  ((sizeof (lispobj))
-                   * (th->control_stack_end-th->control_stack_start)) +
-                  BINDING_STACK_SIZE+ALIEN_STACK_SIZE+dynamic_values_bytes+
-                  32*SIGSTKSZ);
+    free_thread_struct(th);
+}
+
+/* Send the signo to os_thread, retry if the rt signal queue is
+ * full. */
+static int kill_thread_safely(os_thread_t os_thread, int signo)
+{
+    int r;
+    /* The man page does not mention EAGAIN as a valid return value
+     * for either pthread_kill or kill. But that's theory, this is
+     * practice. By waiting here we assume that the delivery of this
+     * signal is not necessary for the delivery of the signals in the
+     * queue. In other words, we _assume_ there are no deadlocks. */
+    while ((r=pthread_kill(os_thread,signo))==EAGAIN) {
+        /* wait a bit then try again in the hope of the rt signal
+         * queue not being full */
+        FSHOW_SIGNAL((stderr,"/rt signal queue full\n"));
+        /* FIXME: some kind of backoff (random, exponential) would be
+         * nice. */
+        sleep(1);
+    }
+    return r;
 }
 
 int interrupt_thread(struct thread *th, lispobj function)
 {
-    /* A thread may also become dead after this test. */
-    if ((th->state != STATE_DEAD)) {
-        /* In clone_threads, if A and B both interrupt C at
-         * approximately the same time, it does not matter: the
-         * second signal will be masked until the handler has
-         * returned from the first one.  In pthreads though, we
-         * can't put the knowledge of what function to call into
-         * the siginfo, so we have to store it in the destination
-         * thread, and do it in such a way that A won't clobber
-         * B's interrupt.  Hence this stupid linked list.
-         *
-         * This does depend on SIG_INTERRUPT_THREAD being queued
-         * (as POSIX RT signals are): we need to keep
-         * interrupt_fun data for exactly as many signals as are
-         * going to be received by the destination thread.
-         */
-        struct cons *c=alloc_cons(function,NIL);
-        int kill_status;
-        /* interrupt_thread_handler locks this spinlock with
-         * interrupts blocked and it does so for the sake of
-         * arrange_return_to_lisp_function, so we must also block
-         * them. */
-        sigset_t newset,oldset;
-        sigemptyset(&newset);
-        sigaddset_blockable(&newset);
-        thread_sigmask(SIG_BLOCK, &newset, &oldset);
-        get_spinlock(&th->interrupt_fun_lock,
-                     (long)arch_os_get_current_thread());
-        kill_status=thread_kill(th->os_thread,SIG_INTERRUPT_THREAD);
-        if(kill_status==0) {
-            ((struct cons *)native_pointer(c))->cdr=th->interrupt_fun;
-            th->interrupt_fun=c;
+    /* In clone_threads, if A and B both interrupt C at approximately
+     * the same time, it does not matter: the second signal will be
+     * masked until the handler has returned from the first one.  In
+     * pthreads though, we can't put the knowledge of what function to
+     * call into the siginfo, so we have to store it in the
+     * destination thread, and do it in such a way that A won't
+     * clobber B's interrupt.  Hence, this stupid linked list.
+     *
+     * This does depend on SIG_INTERRUPT_THREAD being queued (as POSIX
+     * RT signals are): we need to keep interrupt_fun data for exactly
+     * as many signals as are going to be received by the destination
+     * thread.
+     */
+    lispobj c=alloc_cons(function,NIL);
+    sigset_t newset,oldset;
+    sigemptyset(&newset);
+    /* interrupt_thread_handler locks this spinlock with blockables
+     * blocked (it does so for the sake of
+     * arrange_return_to_lisp_function), so we must also block them or
+     * else SIG_STOP_FOR_GC and all_threads_lock will find a way to
+     * deadlock. */
+    sigaddset_blockable(&newset);
+    thread_sigmask(SIG_BLOCK, &newset, &oldset);
+    if (th == arch_os_get_current_thread())
+        lose("cannot interrupt current thread");
+    get_spinlock(&th->interrupt_fun_lock,
+                 (long)arch_os_get_current_thread());
+    ((struct cons *)native_pointer(c))->cdr=th->interrupt_fun;
+    th->interrupt_fun=c;
+    release_spinlock(&th->interrupt_fun_lock);
+    thread_sigmask(SIG_SETMASK,&oldset,0);
+    /* Called from lisp with the thread object as a parameter. Thus,
+     * the object cannot be garbage collected and consequently reaped
+     * and joined. Because it's not joined, kill should work (even if
+     * the thread has died/exited). */
+    {
+        int status=kill_thread_safely(th->os_thread,SIG_INTERRUPT_THREAD);
+        if (status==0) {
+            return 0;
+        } else if (status==ESRCH) {
+            /* This thread has exited. */
+            th->interrupt_fun=NIL;
+            errno=ESRCH;
+            return -1;
+        } else {
+            lose("cannot send SIG_INTERRUPT_THREAD to thread=%lu: %d, %s",
+                 th->os_thread,status,strerror(status));
         }
-        release_spinlock(&th->interrupt_fun_lock);
-        thread_sigmask(SIG_SETMASK,&oldset,0);
-        return (kill_status ? -1 : 0);
     }
-    errno=EPERM; return -1;
 }
 
 /* stopping the world is a two-stage process.  From this thread we signal
@@ -416,24 +472,26 @@ int interrupt_thread(struct thread *th, lispobj function)
 void gc_stop_the_world()
 {
     struct thread *p,*th=arch_os_get_current_thread();
-    FSHOW_SIGNAL((stderr,"/gc_stop_the_world:waiting on lock, thread=%ld\n",
+    int status;
+    FSHOW_SIGNAL((stderr,"/gc_stop_the_world:waiting on lock, thread=%lu\n",
                   th->os_thread));
     /* keep threads from starting while the world is stopped. */
-    get_spinlock(&all_threads_lock,(long)th);
-    FSHOW_SIGNAL((stderr,"/gc_stop_the_world:got lock, thread=%ld\n",
+    pthread_mutex_lock(&all_threads_lock); \
+    FSHOW_SIGNAL((stderr,"/gc_stop_the_world:got lock, thread=%lu\n",
                   th->os_thread));
     /* stop all other threads by sending them SIG_STOP_FOR_GC */
     for(p=all_threads; p; p=p->next) {
         while(p->state==STATE_STARTING) sched_yield();
         if((p!=th) && (p->state==STATE_RUNNING)) {
-            FSHOW_SIGNAL((stderr,"/gc_stop_the_world:sending sig_stop to %ld\n",
+            FSHOW_SIGNAL((stderr,"/gc_stop_the_world: suspending %lu\n",
                           p->os_thread));
-            if(thread_kill(p->os_thread,SIG_STOP_FOR_GC)==-1) {
-                /* we can't kill the thread; assume because it died
-                 * since we last checked */
-                p->state=STATE_DEAD;
-                FSHOW_SIGNAL((stderr,"/gc_stop_the_world:assuming %ld dead\n",
-                   p->os_thread));
+            status=kill_thread_safely(p->os_thread,SIG_STOP_FOR_GC);
+            if (status==ESRCH) {
+                /* This thread has exited. */
+                gc_assert(p->state==STATE_DEAD);
+            } else if (status) {
+                lose("cannot send suspend thread=%lu: %d, %s",
+                     p->os_thread,status,strerror(status));
             }
         }
     }
@@ -445,6 +503,8 @@ void gc_stop_the_world()
         if((p==th) || (p->state==STATE_SUSPENDED) ||
            (p->state==STATE_DEAD)) {
             p=p->next;
+        } else {
+            sched_yield();
         }
     }
     FSHOW_SIGNAL((stderr,"/gc_stop_the_world:end\n"));
@@ -453,6 +513,7 @@ void gc_stop_the_world()
 void gc_start_the_world()
 {
     struct thread *p,*th=arch_os_get_current_thread();
+    int status;
     /* if a resumed thread creates a new thread before we're done with
      * this loop, the new thread will get consed on the front of
      * all_threads, but it won't have been stopped so won't need
@@ -460,23 +521,26 @@ void gc_start_the_world()
     FSHOW_SIGNAL((stderr,"/gc_start_the_world:begin\n"));
     for(p=all_threads;p;p=p->next) {
         gc_assert(p->os_thread!=0);
-       if((p!=th) && (p->state!=STATE_DEAD)) {
+        if((p!=th) && (p->state!=STATE_DEAD)) {
             if(p->state!=STATE_SUSPENDED) {
-                lose("gc_start_the_world: wrong thread state is %ld\n",
+                lose("gc_start_the_world: wrong thread state is %d\n",
                      fixnum_value(p->state));
             }
-            thread_kill(p->os_thread,SIG_STOP_FOR_GC);
-        }
-    }
-    /* we must wait for all threads to leave stopped state else we
-     * risk signal accumulation and lose any meaning of
-     * thread->state */
-    for(p=all_threads;p;) {
-        if((p==th) || (p->state!=STATE_SUSPENDED)) {
-            p=p->next;
+            FSHOW_SIGNAL((stderr, "/gc_start_the_world: resuming %lu\n",
+                          p->os_thread));
+            p->state=STATE_RUNNING;
+            status=kill_thread_safely(p->os_thread,SIG_STOP_FOR_GC);
+            if (status) {
+                lose("cannot resume thread=%lu: %d, %s",
+                     p->os_thread,status,strerror(status));
+            }
         }
     }
-    release_spinlock(&all_threads_lock);
+    /* If we waited here until all threads leave STATE_SUSPENDED, then
+     * SIG_STOP_FOR_GC wouldn't need to be a rt signal. That has some
+     * performance implications, but does away with the 'rt signal
+     * queue full' problem. */
+    pthread_mutex_unlock(&all_threads_lock); \
     FSHOW_SIGNAL((stderr,"/gc_start_the_world:end\n"));
 }
 #endif