darwin_stop_world.c

/*
 * Copyright (c) 1994 by Xerox Corporation.  All rights reserved.
 * Copyright (c) 1996 by Silicon Graphics.  All rights reserved.
 * Copyright (c) 1998 by Fergus Henderson.  All rights reserved.
 * Copyright (c) 2000-2010 by Hewlett-Packard Development Company.
 * All rights reserved.
 * Copyright (c) 2008-2022 Ivan Maidanski
 *
 * THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED
 * OR IMPLIED.  ANY USE IS AT YOUR OWN RISK.
 *
 * Permission is hereby granted to use or copy this program
 * for any purpose, provided the above notices are retained on all copies.
 * Permission to modify the code and to distribute modified code is granted,
 * provided the above notices are retained, and a notice that the code was
 * modified is included with the above copyright notice.
 */

#include "private/pthread_support.h"

/* This probably needs more porting work to ppc64. */

#if defined(GC_DARWIN_THREADS)

#include <sys/sysctl.h>
#include <mach/machine.h>
#include <CoreFoundation/CoreFoundation.h>

/* From "Inside Mac OS X - Mach-O Runtime Architecture" published by Apple
   Page 49:
   "The space beneath the stack pointer, where a new stack frame would normally
   be allocated, is called the red zone. This area as shown in Figure 3-2 may
   be used for any purpose as long as a new stack frame does not need to be
   added to the stack."

   Page 50: "If a leaf procedure's red zone usage would exceed 224 bytes, then
   it must set up a stack frame just like routines that call other routines."
*/
#ifdef POWERPC
# if CPP_WORDSZ == 32
#   define PPC_RED_ZONE_SIZE 224
# elif CPP_WORDSZ == 64
#   define PPC_RED_ZONE_SIZE 320
# endif
#endif

#ifndef DARWIN_DONT_PARSE_STACK

typedef struct StackFrame {
  unsigned long savedSP;
  unsigned long savedCR;
  unsigned long savedLR;
  unsigned long reserved[2];
  unsigned long savedRTOC;
} StackFrame;

GC_INNER ptr_t GC_FindTopOfStack(unsigned long stack_start)
{
  StackFrame *frame = (StackFrame *)stack_start;

  if (stack_start == 0) {
#   ifdef POWERPC
#     if CPP_WORDSZ == 32
        __asm__ __volatile__ ("lwz %0,0(r1)" : "=r" (frame));
#     else
        __asm__ __volatile__ ("ld %0,0(r1)" : "=r" (frame));
#     endif
#   elif defined(ARM32)
        volatile ptr_t sp_reg;
        __asm__ __volatile__ ("mov %0, r7\n" : "=r" (sp_reg));
        frame = (StackFrame *)sp_reg;
#   elif defined(AARCH64)
        volatile ptr_t sp_reg;
        __asm__ __volatile__ ("mov %0, x29\n" : "=r" (sp_reg));
        frame = (StackFrame *)sp_reg;
#   else
#     if defined(CPPCHECK)
        GC_noop1((word)&frame);
#     endif
      ABORT("GC_FindTopOfStack(0) is not implemented");
#   endif
  }

# ifdef DEBUG_THREADS_EXTRA
    GC_log_printf("FindTopOfStack start at sp= %p\n", (void *)frame);
# endif
  while (frame->savedSP != 0) { /* stop if no more stack frames */
    unsigned long maskedLR;

    frame = (StackFrame*)frame->savedSP;

    /* we do these next two checks after going to the next frame
       because the LR for the first stack frame in the loop
       is not set up on purpose, so we shouldn't check it. */
    maskedLR = frame -> savedLR & ~0x3UL;
    if (0 == maskedLR || ~0x3UL == maskedLR)
      break; /* if the next LR is bogus, stop */
  }
# ifdef DEBUG_THREADS_EXTRA
    GC_log_printf("FindTopOfStack finish at sp= %p\n", (void *)frame);
# endif
  return (ptr_t)frame;
}

#endif /* !DARWIN_DONT_PARSE_STACK */

/* GC_query_task_threads controls whether to obtain the list of */
/* the threads from the kernel or to use GC_threads table.      */
#ifdef GC_NO_THREADS_DISCOVERY
# define GC_query_task_threads FALSE
#elif defined(GC_DISCOVER_TASK_THREADS)
# define GC_query_task_threads TRUE
#else
  STATIC GC_bool GC_query_task_threads = FALSE;
#endif /* !GC_NO_THREADS_DISCOVERY */

/* Use implicit threads registration (all task threads excluding the GC */
/* special ones are stopped and scanned).  Should be called before      */
/* GC_INIT() (or, at least, before going multi-threaded).  Deprecated.  */
GC_API void GC_CALL GC_use_threads_discovery(void)
{
# ifdef GC_NO_THREADS_DISCOVERY
    ABORT("Darwin task-threads-based stop and push unsupported");
# else
#   ifndef GC_ALWAYS_MULTITHREADED
      GC_ASSERT(!GC_need_to_lock);
#   endif
#   ifndef GC_DISCOVER_TASK_THREADS
      GC_query_task_threads = TRUE;
#   endif
    GC_init();
# endif
}

#ifndef kCFCoreFoundationVersionNumber_iOS_8_0
# define kCFCoreFoundationVersionNumber_iOS_8_0 1140.1
#endif

/* Evaluates the stack range for a given thread.  Returns the lower     */
/* bound and sets *phi to the upper one.  Sets *pfound_me to TRUE if    */
/* this is current thread, otherwise the value is not changed.          */
STATIC ptr_t GC_stack_range_for(ptr_t *phi, thread_act_t thread, GC_thread p,
                                mach_port_t my_thread, ptr_t *paltstack_lo,
                                ptr_t *paltstack_hi, GC_bool *pfound_me)
{
# ifdef DARWIN_DONT_PARSE_STACK
    GC_stack_context_t crtn;
# endif
  ptr_t lo;

  GC_ASSERT(I_HOLD_LOCK());
  if (thread == my_thread) {
    GC_ASSERT(NULL == p || (p -> flags & DO_BLOCKING) == 0);
    lo = GC_approx_sp();
#   ifndef DARWIN_DONT_PARSE_STACK
      *phi = GC_FindTopOfStack(0);
#   endif
    *pfound_me = TRUE;
  } else if (p != NULL && (p -> flags & DO_BLOCKING) != 0) {
    lo = p -> crtn -> stack_ptr;
#   ifndef DARWIN_DONT_PARSE_STACK
      *phi = p -> crtn -> topOfStack;
#   endif

  } else {
    /* MACHINE_THREAD_STATE_COUNT does not seem to be defined       */
    /* everywhere.  Hence we use our own version.  Alternatively,   */
    /* we could use THREAD_STATE_MAX (but seems to be not optimal). */
    kern_return_t kern_result;
    GC_THREAD_STATE_T state;

#   if defined(ARM32) && defined(ARM_THREAD_STATE32)
      /* Use ARM_UNIFIED_THREAD_STATE on iOS8+ 32-bit targets and on    */
      /* 64-bit H/W (iOS7+ 32-bit mode).                                */
      size_t size;
      static cpu_type_t cputype = 0;

      if (cputype == 0) {
        sysctlbyname("hw.cputype", &cputype, &size, NULL, 0);
      }
      if (cputype == CPU_TYPE_ARM64
          || kCFCoreFoundationVersionNumber
             >= kCFCoreFoundationVersionNumber_iOS_8_0) {
        arm_unified_thread_state_t unified_state;
        mach_msg_type_number_t unified_thread_state_count
                                        = ARM_UNIFIED_THREAD_STATE_COUNT;
#       if defined(CPPCHECK)
#         define GC_ARM_UNIFIED_THREAD_STATE 1
#       else
#         define GC_ARM_UNIFIED_THREAD_STATE ARM_UNIFIED_THREAD_STATE
#       endif
        kern_result = thread_get_state(thread, GC_ARM_UNIFIED_THREAD_STATE,
                                       (natural_t *)&unified_state,
                                       &unified_thread_state_count);
#       if !defined(CPPCHECK)
          if (unified_state.ash.flavor != ARM_THREAD_STATE32) {
            ABORT("unified_state flavor should be ARM_THREAD_STATE32");
          }
#       endif
        state = unified_state;
      } else
#   endif
    /* else */ {
      mach_msg_type_number_t thread_state_count = GC_MACH_THREAD_STATE_COUNT;

      /* Get the thread state (registers, etc.) */
      do {
        kern_result = thread_get_state(thread, GC_MACH_THREAD_STATE,
                                       (natural_t *)&state,
                                       &thread_state_count);
      } while (kern_result == KERN_ABORTED);
    }
#   ifdef DEBUG_THREADS
      GC_log_printf("thread_get_state returns %d\n", kern_result);
#   endif
    if (kern_result != KERN_SUCCESS)
      ABORT("thread_get_state failed");

#   if defined(I386)
      lo = (ptr_t)state.THREAD_FLD(esp);
#     ifndef DARWIN_DONT_PARSE_STACK
        *phi = GC_FindTopOfStack(state.THREAD_FLD(esp));
#     endif
      GC_push_one(state.THREAD_FLD(eax));
      GC_push_one(state.THREAD_FLD(ebx));
      GC_push_one(state.THREAD_FLD(ecx));
      GC_push_one(state.THREAD_FLD(edx));
      GC_push_one(state.THREAD_FLD(edi));
      GC_push_one(state.THREAD_FLD(esi));
      GC_push_one(state.THREAD_FLD(ebp));

#   elif defined(X86_64)
      lo = (ptr_t)state.THREAD_FLD(rsp);
#     ifndef DARWIN_DONT_PARSE_STACK
        *phi = GC_FindTopOfStack(state.THREAD_FLD(rsp));
#     endif
      GC_push_one(state.THREAD_FLD(rax));
      GC_push_one(state.THREAD_FLD(rbx));
      GC_push_one(state.THREAD_FLD(rcx));
      GC_push_one(state.THREAD_FLD(rdx));
      GC_push_one(state.THREAD_FLD(rdi));
      GC_push_one(state.THREAD_FLD(rsi));
      GC_push_one(state.THREAD_FLD(rbp));
      /* rsp is skipped.        */
      GC_push_one(state.THREAD_FLD(r8));
      GC_push_one(state.THREAD_FLD(r9));
      GC_push_one(state.THREAD_FLD(r10));
      GC_push_one(state.THREAD_FLD(r11));
      GC_push_one(state.THREAD_FLD(r12));
      GC_push_one(state.THREAD_FLD(r13));
      GC_push_one(state.THREAD_FLD(r14));
      GC_push_one(state.THREAD_FLD(r15));

#   elif defined(POWERPC)
      lo = (ptr_t)(state.THREAD_FLD(r1) - PPC_RED_ZONE_SIZE);
#     ifndef DARWIN_DONT_PARSE_STACK
        *phi = GC_FindTopOfStack(state.THREAD_FLD(r1));
#     endif
      GC_push_one(state.THREAD_FLD(r0));
      /* r1 is skipped. */
      GC_push_one(state.THREAD_FLD(r2));
      GC_push_one(state.THREAD_FLD(r3));
      GC_push_one(state.THREAD_FLD(r4));
      GC_push_one(state.THREAD_FLD(r5));
      GC_push_one(state.THREAD_FLD(r6));
      GC_push_one(state.THREAD_FLD(r7));
      GC_push_one(state.THREAD_FLD(r8));
      GC_push_one(state.THREAD_FLD(r9));
      GC_push_one(state.THREAD_FLD(r10));
      GC_push_one(state.THREAD_FLD(r11));
      GC_push_one(state.THREAD_FLD(r12));
      GC_push_one(state.THREAD_FLD(r13));
      GC_push_one(state.THREAD_FLD(r14));
      GC_push_one(state.THREAD_FLD(r15));
      GC_push_one(state.THREAD_FLD(r16));
      GC_push_one(state.THREAD_FLD(r17));
      GC_push_one(state.THREAD_FLD(r18));
      GC_push_one(state.THREAD_FLD(r19));
      GC_push_one(state.THREAD_FLD(r20));
      GC_push_one(state.THREAD_FLD(r21));
      GC_push_one(state.THREAD_FLD(r22));
      GC_push_one(state.THREAD_FLD(r23));
      GC_push_one(state.THREAD_FLD(r24));
      GC_push_one(state.THREAD_FLD(r25));
      GC_push_one(state.THREAD_FLD(r26));
      GC_push_one(state.THREAD_FLD(r27));
      GC_push_one(state.THREAD_FLD(r28));
      GC_push_one(state.THREAD_FLD(r29));
      GC_push_one(state.THREAD_FLD(r30));
      GC_push_one(state.THREAD_FLD(r31));

#   elif defined(ARM32)
      lo = (ptr_t)state.THREAD_FLD(sp);
#     ifndef DARWIN_DONT_PARSE_STACK
        *phi = GC_FindTopOfStack(state.THREAD_FLD(r[7])); /* fp */
#     endif
      {
        int j;
        for (j = 0; j < 7; j++)
          GC_push_one(state.THREAD_FLD(r[j]));
        j++; /* "r7" is skipped (iOS uses it as a frame pointer) */
        for (; j <= 12; j++)
          GC_push_one(state.THREAD_FLD(r[j]));
      }
      /* "cpsr", "pc" and "sp" are skipped */
      GC_push_one(state.THREAD_FLD(lr));

#   elif defined(AARCH64)
      lo = (ptr_t)state.THREAD_FLD(sp);
#     ifndef DARWIN_DONT_PARSE_STACK
        *phi = GC_FindTopOfStack(state.THREAD_FLD(fp));
#     endif
      {
        int j;
        for (j = 0; j <= 28; j++) {
          GC_push_one(state.THREAD_FLD(x[j]));
        }
      }
      /* "cpsr", "fp", "pc" and "sp" are skipped */
      GC_push_one(state.THREAD_FLD(lr));

#   elif defined(CPPCHECK)
      lo = NULL;
#   else
#     error FIXME for non-arm/ppc/x86 architectures
#   endif
  } /* thread != my_thread */

# ifndef DARWIN_DONT_PARSE_STACK
    /* TODO: Determine p and handle altstack if !DARWIN_DONT_PARSE_STACK */
    UNUSED_ARG(paltstack_hi);
# else
    /* p is guaranteed to be non-NULL regardless of GC_query_task_threads. */
#   ifdef CPPCHECK
      if (NULL == p) ABORT("Bad GC_stack_range_for call");
#   endif
    crtn = p -> crtn;
    *phi = crtn -> stack_end;
    if (crtn -> altstack != NULL && (word)(crtn -> altstack) <= (word)lo
        && (word)lo <= (word)(crtn -> altstack) + crtn -> altstack_size) {
      *paltstack_lo = lo;
      *paltstack_hi = crtn -> altstack + crtn -> altstack_size;
      lo = crtn -> normstack;
      *phi = lo + crtn -> normstack_size;
    } else
# endif
  /* else */ {
    *paltstack_lo = NULL;
  }
# if defined(STACKPTR_CORRECTOR_AVAILABLE) && defined(DARWIN_DONT_PARSE_STACK)
    if (GC_sp_corrector != 0)
      GC_sp_corrector((void **)&lo, (void *)(p -> id));
# endif
# ifdef DEBUG_THREADS
    GC_log_printf("Darwin: Stack for thread %p is [%p,%p)\n",
                  (void *)(word)thread, (void *)lo, (void *)(*phi));
# endif
  return lo;
}

GC_INNER void GC_push_all_stacks(void)
{
  ptr_t hi, altstack_lo, altstack_hi;
  task_t my_task = current_task();
  mach_port_t my_thread = mach_thread_self();
  GC_bool found_me = FALSE;
  int nthreads = 0;
  word total_size = 0;

  GC_ASSERT(I_HOLD_LOCK());
  GC_ASSERT(GC_thr_initialized);

# ifndef DARWIN_DONT_PARSE_STACK
    if (GC_query_task_threads) {
      int i;
      kern_return_t kern_result;
      thread_act_array_t act_list;
      mach_msg_type_number_t listcount;

      /* Obtain the list of the threads from the kernel.  */
      kern_result = task_threads(my_task, &act_list, &listcount);
      if (kern_result != KERN_SUCCESS)
        ABORT("task_threads failed");

      for (i = 0; i < (int)listcount; i++) {
        thread_act_t thread = act_list[i];
        ptr_t lo = GC_stack_range_for(&hi, thread, NULL, my_thread,
                                      &altstack_lo, &altstack_hi, &found_me);

        if (lo) {
          GC_ASSERT((word)lo <= (word)hi);
          total_size += hi - lo;
          GC_push_all_stack(lo, hi);
        }
        /* TODO: Handle altstack */
        nthreads++;
        mach_port_deallocate(my_task, thread);
      } /* for (i=0; ...) */

      vm_deallocate(my_task, (vm_address_t)act_list,
                    sizeof(thread_t) * listcount);
    } else
# endif /* !DARWIN_DONT_PARSE_STACK */
  /* else */ {
    int i;

    for (i = 0; i < THREAD_TABLE_SZ; i++) {
      GC_thread p;

      for (p = GC_threads[i]; p != NULL; p = p -> tm.next) {
        GC_ASSERT(THREAD_TABLE_INDEX(p -> id) == i);
        if (!KNOWN_FINISHED(p)) {
          thread_act_t thread = (thread_act_t)(p -> mach_thread);
          ptr_t lo = GC_stack_range_for(&hi, thread, p, my_thread,
                                        &altstack_lo, &altstack_hi, &found_me);

          if (lo) {
            GC_ASSERT((word)lo <= (word)hi);
            total_size += hi - lo;
            GC_push_all_stack_sections(lo, hi, p -> crtn -> traced_stack_sect);
          }
          if (altstack_lo) {
            total_size += altstack_hi - altstack_lo;
            GC_push_all_stack(altstack_lo, altstack_hi);
          }
          nthreads++;
        }
      }
    } /* for (i=0; ...) */
  }

  mach_port_deallocate(my_task, my_thread);
  GC_VERBOSE_LOG_PRINTF("Pushed %d thread stacks\n", nthreads);
  if (!found_me && !GC_in_thread_creation)
    ABORT("Collecting from unknown thread");
  GC_total_stacksize = total_size;
}

#ifndef GC_NO_THREADS_DISCOVERY

# ifdef MPROTECT_VDB
    STATIC mach_port_t GC_mach_handler_thread = 0;
    STATIC GC_bool GC_use_mach_handler_thread = FALSE;

    GC_INNER void GC_darwin_register_self_mach_handler(void)
    {
      GC_mach_handler_thread = mach_thread_self();
      GC_use_mach_handler_thread = TRUE;
    }
# endif /* MPROTECT_VDB */

# ifndef GC_MAX_MACH_THREADS
#   define GC_MAX_MACH_THREADS THREAD_TABLE_SZ
# endif

  struct GC_mach_thread {
    thread_act_t thread;
    GC_bool suspended;
  };

  struct GC_mach_thread GC_mach_threads[GC_MAX_MACH_THREADS];
  STATIC int GC_mach_threads_count = 0;
  /* FIXME: it is better to implement GC_mach_threads as a hash set.  */

/* returns true if there's a thread in act_list that wasn't in old_list */
STATIC GC_bool GC_suspend_thread_list(thread_act_array_t act_list, int count,
                                      thread_act_array_t old_list,
                                      int old_count, task_t my_task,
                                      mach_port_t my_thread)
{
  int i;
  int j = -1;
  GC_bool changed = FALSE;

  GC_ASSERT(I_HOLD_LOCK());
  for (i = 0; i < count; i++) {
    thread_act_t thread = act_list[i];
    GC_bool found;
    kern_return_t kern_result;

    if (thread == my_thread
#       ifdef MPROTECT_VDB
          || (GC_mach_handler_thread == thread && GC_use_mach_handler_thread)
#       endif
#       ifdef PARALLEL_MARK
          || GC_is_mach_marker(thread) /* ignore the parallel markers */
#       endif
        ) {
      /* Do not add our one, parallel marker and the handler threads;   */
      /* consider it as found (e.g., it was processed earlier).         */
      mach_port_deallocate(my_task, thread);
      continue;
    }

    /* find the current thread in the old list */
    found = FALSE;
    {
      int last_found = j; /* remember the previous found thread index */

      /* Search for the thread starting from the last found one first.  */
      while (++j < old_count)
        if (old_list[j] == thread) {
          found = TRUE;
          break;
        }
      if (!found) {
        /* If not found, search in the rest (beginning) of the list.    */
        for (j = 0; j < last_found; j++)
          if (old_list[j] == thread) {
            found = TRUE;
            break;
          }
      }
    }

    if (found) {
      /* It is already in the list, skip processing, release mach port. */
      mach_port_deallocate(my_task, thread);
      continue;
    }

    /* add it to the GC_mach_threads list */
    if (GC_mach_threads_count == GC_MAX_MACH_THREADS)
      ABORT("Too many threads");
    GC_mach_threads[GC_mach_threads_count].thread = thread;
    /* default is not suspended */
    GC_mach_threads[GC_mach_threads_count].suspended = FALSE;
    changed = TRUE;

#   ifdef DEBUG_THREADS
      GC_log_printf("Suspending %p\n", (void *)(word)thread);
#   endif
    /* Unconditionally suspend the thread.  It will do no     */
    /* harm if it is already suspended by the client logic.   */
    GC_acquire_dirty_lock();
    do {
      kern_result = thread_suspend(thread);
    } while (kern_result == KERN_ABORTED);
    GC_release_dirty_lock();
    if (kern_result != KERN_SUCCESS) {
      /* The thread may have quit since the thread_threads() call we  */
      /* mark already suspended so it's not dealt with anymore later. */
      GC_mach_threads[GC_mach_threads_count].suspended = FALSE;
    } else {
      /* Mark the thread as suspended and require resume.     */
      GC_mach_threads[GC_mach_threads_count].suspended = TRUE;
      if (GC_on_thread_event)
        GC_on_thread_event(GC_EVENT_THREAD_SUSPENDED, (void *)(word)thread);
    }
    GC_mach_threads_count++;
  }
  return changed;
}

#endif /* !GC_NO_THREADS_DISCOVERY */

GC_INNER void GC_stop_world(void)
{
  task_t my_task = current_task();
  mach_port_t my_thread = mach_thread_self();
  kern_return_t kern_result;

  GC_ASSERT(I_HOLD_LOCK());
  GC_ASSERT(GC_thr_initialized);
# ifdef DEBUG_THREADS
    GC_log_printf("Stopping the world from thread %p\n",
                  (void *)(word)my_thread);
# endif
# ifdef PARALLEL_MARK
    if (GC_parallel) {
      GC_acquire_mark_lock();
      GC_ASSERT(GC_fl_builder_count == 0);
      /* We should have previously waited for it to become zero. */
    }
# endif /* PARALLEL_MARK */

  if (GC_query_task_threads) {
#   ifndef GC_NO_THREADS_DISCOVERY
      GC_bool changed;
      thread_act_array_t act_list, prev_list;
      mach_msg_type_number_t listcount, prevcount;

      /* Clear out the mach threads list table.  We do not need to      */
      /* really clear GC_mach_threads[] as it is used only in the range */
      /* from 0 to GC_mach_threads_count-1, inclusive.                  */
      GC_mach_threads_count = 0;

      /* Loop stopping threads until you have gone over the whole list  */
      /* twice without a new one appearing.  thread_create() won't      */
      /* return (and thus the thread stop) until the new thread exists, */
      /* so there is no window whereby you could stop a thread,         */
      /* recognize it is stopped, but then have a new thread it created */
      /* before stopping show up later.                                 */
      changed = TRUE;
      prev_list = NULL;
      prevcount = 0;
      do {
        kern_result = task_threads(my_task, &act_list, &listcount);

        if (kern_result == KERN_SUCCESS) {
          changed = GC_suspend_thread_list(act_list, listcount, prev_list,
                                           prevcount, my_task, my_thread);

          if (prev_list != NULL) {
            /* Thread ports are not deallocated by list, unused ports   */
            /* deallocated in GC_suspend_thread_list, used - kept in    */
            /* GC_mach_threads till GC_start_world as otherwise thread  */
            /* object change can occur and GC_start_world will not      */
            /* find the thread to resume which will cause app to hang.  */
            vm_deallocate(my_task, (vm_address_t)prev_list,
                          sizeof(thread_t) * prevcount);
          }

          /* Repeat while having changes. */
          prev_list = act_list;
          prevcount = listcount;
        }
      } while (changed);

      GC_ASSERT(prev_list != 0);
      /* The thread ports are not deallocated by list, see above.       */
      vm_deallocate(my_task, (vm_address_t)act_list,
                    sizeof(thread_t) * listcount);
#   endif /* !GC_NO_THREADS_DISCOVERY */

  } else {
    unsigned i;

    for (i = 0; i < THREAD_TABLE_SZ; i++) {
      GC_thread p;

      for (p = GC_threads[i]; p != NULL; p = p -> tm.next) {
        if ((p -> flags & (FINISHED | DO_BLOCKING)) == 0
            && p -> mach_thread != my_thread) {
          GC_acquire_dirty_lock();
          do {
            kern_result = thread_suspend(p -> mach_thread);
          } while (kern_result == KERN_ABORTED);
          GC_release_dirty_lock();
          if (kern_result != KERN_SUCCESS)
            ABORT("thread_suspend failed");
          if (GC_on_thread_event)
            GC_on_thread_event(GC_EVENT_THREAD_SUSPENDED,
                               (void *)(word)(p -> mach_thread));
        }
      }
    }
  }

# ifdef MPROTECT_VDB
    if (GC_auto_incremental) {
      GC_mprotect_stop();
    }
# endif
# ifdef PARALLEL_MARK
    if (GC_parallel)
      GC_release_mark_lock();
# endif

# ifdef DEBUG_THREADS
    GC_log_printf("World stopped from %p\n", (void *)(word)my_thread);
# endif
  mach_port_deallocate(my_task, my_thread);
}

GC_INLINE void GC_thread_resume(thread_act_t thread)
{
  kern_return_t kern_result;
# if defined(DEBUG_THREADS) || defined(GC_ASSERTIONS)
    struct thread_basic_info info;
    mach_msg_type_number_t outCount = THREAD_BASIC_INFO_COUNT;

#   ifdef CPPCHECK
      info.run_state = 0;
#   endif
    kern_result = thread_info(thread, THREAD_BASIC_INFO,
                              (thread_info_t)&info, &outCount);
    if (kern_result != KERN_SUCCESS)
      ABORT("thread_info failed");
# endif
  GC_ASSERT(I_HOLD_LOCK());
# ifdef DEBUG_THREADS
    GC_log_printf("Resuming thread %p with state %d\n", (void *)(word)thread,
                  info.run_state);
# endif
  /* Resume the thread */
  kern_result = thread_resume(thread);
  if (kern_result != KERN_SUCCESS) {
    WARN("thread_resume(%p) failed: mach port invalid\n", thread);
  } else if (GC_on_thread_event) {
    GC_on_thread_event(GC_EVENT_THREAD_UNSUSPENDED, (void *)(word)thread);
  }
}

GC_INNER void GC_start_world(void)
{
  task_t my_task = current_task();

  GC_ASSERT(I_HOLD_LOCK()); /* held continuously since the world stopped */
# ifdef DEBUG_THREADS
    GC_log_printf("World starting\n");
# endif
# ifdef MPROTECT_VDB
    if (GC_auto_incremental) {
      GC_mprotect_resume();
    }
# endif

  if (GC_query_task_threads) {
#   ifndef GC_NO_THREADS_DISCOVERY
      int i, j;
      kern_return_t kern_result;
      thread_act_array_t act_list;
      mach_msg_type_number_t listcount;

      kern_result = task_threads(my_task, &act_list, &listcount);
      if (kern_result != KERN_SUCCESS)
        ABORT("task_threads failed");

      j = (int)listcount;
      for (i = 0; i < GC_mach_threads_count; i++) {
        thread_act_t thread = GC_mach_threads[i].thread;

        if (GC_mach_threads[i].suspended) {
          int last_found = j;   /* The thread index found during the    */
                                /* previous iteration (count value      */
                                /* means no thread found yet).          */

          /* Search for the thread starting from the last found one first. */
          while (++j < (int)listcount) {
            if (act_list[j] == thread)
              break;
          }
          if (j >= (int)listcount) {
            /* If not found, search in the rest (beginning) of the list. */
            for (j = 0; j < last_found; j++) {
              if (act_list[j] == thread)
                break;
            }
          }
          if (j != last_found) {
            /* The thread is alive, resume it.  */
            GC_thread_resume(thread);
          }
        } else {
          /* This thread was failed to be suspended by GC_stop_world,   */
          /* no action needed.                                          */
#         ifdef DEBUG_THREADS
            GC_log_printf("Not resuming thread %p as it is not suspended\n",
                          (void *)(word)thread);
#         endif
        }
        mach_port_deallocate(my_task, thread);
      }

      for (i = 0; i < (int)listcount; i++)
        mach_port_deallocate(my_task, act_list[i]);
      vm_deallocate(my_task, (vm_address_t)act_list,
                    sizeof(thread_t) * listcount);
#   endif /* !GC_NO_THREADS_DISCOVERY */

  } else {
    int i;
    mach_port_t my_thread = mach_thread_self();

    for (i = 0; i < THREAD_TABLE_SZ; i++) {
      GC_thread p;

      for (p = GC_threads[i]; p != NULL; p = p -> tm.next) {
        if ((p -> flags & (FINISHED | DO_BLOCKING)) == 0
            && p -> mach_thread != my_thread)
          GC_thread_resume(p -> mach_thread);
      }
    }

    mach_port_deallocate(my_task, my_thread);
  }

# ifdef DEBUG_THREADS
    GC_log_printf("World started\n");
# endif
}

#endif /* GC_DARWIN_THREADS */