libgo/runtime/proc.c

   1 // Copyright 2009 The Go Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 #include <errno.h>
   6 #include <limits.h>
   7 #include <signal.h>
   8 #include <stdlib.h>
   9 #include <pthread.h>
  10 #include <unistd.h>
  11
  12 #include "config.h"
  13
  14 #ifdef HAVE_DL_ITERATE_PHDR
  15 #include <link.h>
  16 #endif
  17
  18 #include "runtime.h"
  19 #include "arch.h"
  20 #include "defs.h"
  21 #include "go-type.h"
  22
  23 #ifdef USING_SPLIT_STACK
  24
  25 /* FIXME: These are not declared anywhere.  */
  26
  27 extern void __splitstack_getcontext(void *context[10]);
  28
  29 extern void __splitstack_setcontext(void *context[10]);
  30
  31 extern void *__splitstack_makecontext(size_t, void *context[10], size_t *);
  32
  33 extern void * __splitstack_resetcontext(void *context[10], size_t *);
  34
  35 extern void *__splitstack_find(void *, void *, size_t *, void **, void **,
  36                                void **);
  37
  38 extern void __splitstack_block_signals (int *, int *);
  39
  40 extern void __splitstack_block_signals_context (void *context[10], int *,
  41                                                 int *);
  42
  43 #endif
  44
  45 #ifndef PTHREAD_STACK_MIN
  46 # define PTHREAD_STACK_MIN 8192
  47 #endif
  48
  49 #if defined(USING_SPLIT_STACK) && defined(LINKER_SUPPORTS_SPLIT_STACK)
  50 # define StackMin PTHREAD_STACK_MIN
  51 #else
  52 # define StackMin ((sizeof(char *) < 8) ? 2 * 1024 * 1024 : 4 * 1024 * 1024)
  53 #endif
  54
  55 uintptr runtime_stacks_sys;
  56
  57 void gtraceback(G*)
  58   __asm__(GOSYM_PREFIX "runtime.gtraceback");
  59
  60 #ifdef __rtems__
  61 #define __thread
  62 #endif
  63
  64 static __thread G *g;
  65
  66 #ifndef SETCONTEXT_CLOBBERS_TLS
  67
  68 static inline void
  69 initcontext(void)
  70 {
  71 }
  72
  73 static inline void
  74 fixcontext(ucontext_t *c __attribute__ ((unused)))
  75 {
  76 }
  77
  78 #else
  79
  80 # if defined(__x86_64__) && defined(__sun__)
  81
  82 // x86_64 Solaris 10 and 11 have a bug: setcontext switches the %fs
  83 // register to that of the thread which called getcontext.  The effect
  84 // is that the address of all __thread variables changes.  This bug
  85 // also affects pthread_self() and pthread_getspecific.  We work
  86 // around it by clobbering the context field directly to keep %fs the
  87 // same.
  88
  89 static __thread greg_t fs;
  90
  91 static inline void
  92 initcontext(void)
  93 {
  94         ucontext_t c;
  95
  96         getcontext(&c);
  97         fs = c.uc_mcontext.gregs[REG_FSBASE];
  98 }
  99
 100 static inline void
 101 fixcontext(ucontext_t* c)
 102 {
 103         c->uc_mcontext.gregs[REG_FSBASE] = fs;
 104 }
 105
 106 # elif defined(__NetBSD__)
 107
 108 // NetBSD has a bug: setcontext clobbers tlsbase, we need to save
 109 // and restore it ourselves.
 110
 111 static __thread __greg_t tlsbase;
 112
 113 static inline void
 114 initcontext(void)
 115 {
 116         ucontext_t c;
 117
 118         getcontext(&c);
 119         tlsbase = c.uc_mcontext._mc_tlsbase;
 120 }
 121
 122 static inline void
 123 fixcontext(ucontext_t* c)
 124 {
 125         c->uc_mcontext._mc_tlsbase = tlsbase;
 126 }
 127
 128 # elif defined(__sparc__)
 129
 130 static inline void
 131 initcontext(void)
 132 {
 133 }
 134
 135 static inline void
 136 fixcontext(ucontext_t *c)
 137 {
 138         /* ??? Using
 139              register unsigned long thread __asm__("%g7");
 140              c->uc_mcontext.gregs[REG_G7] = thread;
 141            results in
 142              error: variable ‘thread’ might be clobbered by \
 143                 ‘longjmp’ or ‘vfork’ [-Werror=clobbered]
 144            which ought to be false, as %g7 is a fixed register.  */
 145
 146         if (sizeof (c->uc_mcontext.gregs[REG_G7]) == 8)
 147                 asm ("stx %%g7, %0" : "=m"(c->uc_mcontext.gregs[REG_G7]));
 148         else
 149                 asm ("st %%g7, %0" : "=m"(c->uc_mcontext.gregs[REG_G7]));
 150 }
 151
 152 # elif defined(_AIX)
 153
 154 static inline void
 155 initcontext(void)
 156 {
 157 }
 158
 159 static inline void
 160 fixcontext(ucontext_t* c)
 161 {
 162         // Thread pointer is in r13, per 64-bit ABI.
 163         if (sizeof (c->uc_mcontext.jmp_context.gpr[13]) == 8)
 164                 asm ("std 13, %0" : "=m"(c->uc_mcontext.jmp_context.gpr[13]));
 165 }
 166
 167 # else
 168
 169 #  error unknown case for SETCONTEXT_CLOBBERS_TLS
 170
 171 # endif
 172
 173 #endif
 174
 175 // ucontext_arg returns a properly aligned ucontext_t value.  On some
 176 // systems a ucontext_t value must be aligned to a 16-byte boundary.
 177 // The g structure that has fields of type ucontext_t is defined in
 178 // Go, and Go has no simple way to align a field to such a boundary.
 179 // So we make the field larger in runtime2.go and pick an appropriate
 180 // offset within the field here.
 181 static ucontext_t*
 182 ucontext_arg(uintptr* go_ucontext)
 183 {
 184         uintptr_t p = (uintptr_t)go_ucontext;
 185         size_t align = __alignof__(ucontext_t);
 186         if(align > 16) {
 187                 // We only ensured space for up to a 16 byte alignment
 188                 // in libgo/go/runtime/runtime2.go.
 189                 runtime_throw("required alignment of ucontext_t too large");
 190         }
 191         p = (p + align - 1) &~ (uintptr_t)(align - 1);
 192         return (ucontext_t*)p;
 193 }
 194
 195 // We can not always refer to the TLS variables directly.  The
 196 // compiler will call tls_get_addr to get the address of the variable,
 197 // and it may hold it in a register across a call to schedule.  When
 198 // we get back from the call we may be running in a different thread,
 199 // in which case the register now points to the TLS variable for a
 200 // different thread.  We use non-inlinable functions to avoid this
 201 // when necessary.
 202
 203 G* runtime_g(void) __attribute__ ((noinline, no_split_stack));
 204
 205 G*
 206 runtime_g(void)
 207 {
 208         return g;
 209 }
 210
 211 M* runtime_m(void) __attribute__ ((noinline, no_split_stack));
 212
 213 M*
 214 runtime_m(void)
 215 {
 216         if(g == nil)
 217                 return nil;
 218         return g->m;
 219 }
 220
 221 // Set g.
 222 void
 223 runtime_setg(G* gp)
 224 {
 225         g = gp;
 226 }
 227
 228 void runtime_newosproc(M *)
 229   __asm__(GOSYM_PREFIX "runtime.newosproc");
 230
 231 // Start a new thread.
 232 void
 233 runtime_newosproc(M *mp)
 234 {
 235         pthread_attr_t attr;
 236         sigset_t clear, old;
 237         pthread_t tid;
 238         int tries;
 239         int ret;
 240
 241         if(pthread_attr_init(&attr) != 0)
 242                 runtime_throw("pthread_attr_init");
 243         if(pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED) != 0)
 244                 runtime_throw("pthread_attr_setdetachstate");
 245
 246         // Block signals during pthread_create so that the new thread
 247         // starts with signals disabled.  It will enable them in minit.
 248         sigfillset(&clear);
 249
 250 #ifdef SIGTRAP
 251         // Blocking SIGTRAP reportedly breaks gdb on Alpha GNU/Linux.
 252         sigdelset(&clear, SIGTRAP);
 253 #endif
 254
 255         sigemptyset(&old);
 256         pthread_sigmask(SIG_BLOCK, &clear, &old);
 257
 258         for (tries = 0; tries < 20; tries++) {
 259                 ret = pthread_create(&tid, &attr, runtime_mstart, mp);
 260                 if (ret != EAGAIN) {
 261                         break;
 262                 }
 263                 runtime_usleep((tries + 1) * 1000); // Milliseconds.
 264         }
 265
 266         pthread_sigmask(SIG_SETMASK, &old, nil);
 267
 268         if (ret != 0) {
 269                 runtime_printf("pthread_create failed: %d\n", ret);
 270                 runtime_throw("pthread_create");
 271         }
 272 }
 273
 274 // Switch context to a different goroutine.  This is like longjmp.
 275 void runtime_gogo(G*) __attribute__ ((noinline));
 276 void
 277 runtime_gogo(G* newg)
 278 {
 279 #ifdef USING_SPLIT_STACK
 280         __splitstack_setcontext((void*)(&newg->stackcontext[0]));
 281 #endif
 282         g = newg;
 283         newg->fromgogo = true;
 284         fixcontext(ucontext_arg(&newg->context[0]));
 285         setcontext(ucontext_arg(&newg->context[0]));
 286         runtime_throw("gogo setcontext returned");
 287 }
 288
 289 // Save context and call fn passing g as a parameter.  This is like
 290 // setjmp.  Because getcontext always returns 0, unlike setjmp, we use
 291 // g->fromgogo as a code.  It will be true if we got here via
 292 // setcontext.  g == nil the first time this is called in a new m.
 293 void runtime_mcall(FuncVal *) __attribute__ ((noinline));
 294 void
 295 runtime_mcall(FuncVal *fv)
 296 {
 297         M *mp;
 298         G *gp;
 299 #ifndef USING_SPLIT_STACK
 300         void *afterregs;
 301 #endif
 302
 303         // Ensure that all registers are on the stack for the garbage
 304         // collector.
 305         __builtin_unwind_init();
 306
 307         gp = g;
 308         mp = gp->m;
 309         if(gp == mp->g0)
 310                 runtime_throw("runtime: mcall called on m->g0 stack");
 311
 312         if(gp != nil) {
 313
 314 #ifdef USING_SPLIT_STACK
 315                 __splitstack_getcontext((void*)(&g->stackcontext[0]));
 316 #else
 317                 // We have to point to an address on the stack that is
 318                 // below the saved registers.
 319                 gp->gcnextsp = (uintptr)(&afterregs);
 320 #endif
 321                 gp->fromgogo = false;
 322                 getcontext(ucontext_arg(&gp->context[0]));
 323
 324                 // When we return from getcontext, we may be running
 325                 // in a new thread.  That means that g may have
 326                 // changed.  It is a global variables so we will
 327                 // reload it, but the address of g may be cached in
 328                 // our local stack frame, and that address may be
 329                 // wrong.  Call the function to reload the value for
 330                 // this thread.
 331                 gp = runtime_g();
 332                 mp = gp->m;
 333
 334                 if(gp->traceback != nil)
 335                         gtraceback(gp);
 336         }
 337         if (gp == nil || !gp->fromgogo) {
 338 #ifdef USING_SPLIT_STACK
 339                 __splitstack_setcontext((void*)(&mp->g0->stackcontext[0]));
 340 #endif
 341                 mp->g0->entry = fv;
 342                 mp->g0->param = gp;
 343
 344                 // It's OK to set g directly here because this case
 345                 // can not occur if we got here via a setcontext to
 346                 // the getcontext call just above.
 347                 g = mp->g0;
 348
 349                 fixcontext(ucontext_arg(&mp->g0->context[0]));
 350                 setcontext(ucontext_arg(&mp->g0->context[0]));
 351                 runtime_throw("runtime: mcall function returned");
 352         }
 353 }
 354
 355 // Goroutine scheduler
 356 // The scheduler's job is to distribute ready-to-run goroutines over worker threads.
 357 //
 358 // The main concepts are:
 359 // G - goroutine.
 360 // M - worker thread, or machine.
 361 // P - processor, a resource that is required to execute Go code.
 362 //     M must have an associated P to execute Go code, however it can be
 363 //     blocked or in a syscall w/o an associated P.
 364 //
 365 // Design doc at http://golang.org/s/go11sched.
 366
 367 extern bool* runtime_getCgoHasExtraM()
 368   __asm__ (GOSYM_PREFIX "runtime.getCgoHasExtraM");
 369 extern G* allocg(void)
 370   __asm__ (GOSYM_PREFIX "runtime.allocg");
 371
 372 Sched*  runtime_sched;
 373 int32   runtime_ncpu;
 374
 375 bool    runtime_isarchive;
 376
 377 extern void kickoff(void)
 378   __asm__(GOSYM_PREFIX "runtime.kickoff");
 379 extern void mstart1(void)
 380   __asm__(GOSYM_PREFIX "runtime.mstart1");
 381 extern void stopm(void)
 382   __asm__(GOSYM_PREFIX "runtime.stopm");
 383 extern void handoffp(P*)
 384   __asm__(GOSYM_PREFIX "runtime.handoffp");
 385 extern void wakep(void)
 386   __asm__(GOSYM_PREFIX "runtime.wakep");
 387 extern void stoplockedm(void)
 388   __asm__(GOSYM_PREFIX "runtime.stoplockedm");
 389 extern void schedule(void)
 390   __asm__(GOSYM_PREFIX "runtime.schedule");
 391 extern void execute(G*, bool)
 392   __asm__(GOSYM_PREFIX "runtime.execute");
 393 extern void reentersyscall(uintptr, uintptr)
 394   __asm__(GOSYM_PREFIX "runtime.reentersyscall");
 395 extern void reentersyscallblock(uintptr, uintptr)
 396   __asm__(GOSYM_PREFIX "runtime.reentersyscallblock");
 397 extern G* gfget(P*)
 398   __asm__(GOSYM_PREFIX "runtime.gfget");
 399 extern void acquirep(P*)
 400   __asm__(GOSYM_PREFIX "runtime.acquirep");
 401 extern P* releasep(void)
 402   __asm__(GOSYM_PREFIX "runtime.releasep");
 403 extern void incidlelocked(int32)
 404   __asm__(GOSYM_PREFIX "runtime.incidlelocked");
 405 extern void globrunqput(G*)
 406   __asm__(GOSYM_PREFIX "runtime.globrunqput");
 407 extern P* pidleget(void)
 408   __asm__(GOSYM_PREFIX "runtime.pidleget");
 409 extern struct mstats* getMemstats(void)
 410   __asm__(GOSYM_PREFIX "runtime.getMemstats");
 411
 412 bool runtime_isstarted;
 413
 414 // Used to determine the field alignment.
 415
 416 struct field_align
 417 {
 418   char c;
 419   Hchan *p;
 420 };
 421
 422 void getTraceback(G*, G*) __asm__(GOSYM_PREFIX "runtime.getTraceback");
 423
 424 // getTraceback stores a traceback of gp in the g's traceback field
 425 // and then returns to me.  We expect that gp's traceback is not nil.
 426 // It works by saving me's current context, and checking gp's traceback field.
 427 // If gp's traceback field is not nil, it starts running gp.
 428 // In places where we call getcontext, we check the traceback field.
 429 // If it is not nil, we collect a traceback, and then return to the
 430 // goroutine stored in the traceback field, which is me.
 431 void getTraceback(G* me, G* gp)
 432 {
 433 #ifdef USING_SPLIT_STACK
 434         __splitstack_getcontext((void*)(&me->stackcontext[0]));
 435 #endif
 436         getcontext(ucontext_arg(&me->context[0]));
 437
 438         if (gp->traceback != nil) {
 439                 runtime_gogo(gp);
 440         }
 441 }
 442
 443 // Do a stack trace of gp, and then restore the context to
 444 // gp->traceback->gp.
 445
 446 void
 447 gtraceback(G* gp)
 448 {
 449         Traceback* traceback;
 450         M* holdm;
 451
 452         traceback = gp->traceback;
 453         gp->traceback = nil;
 454         holdm = gp->m;
 455         if(holdm != nil && holdm != g->m)
 456                 runtime_throw("gtraceback: m is not nil");
 457         gp->m = traceback->gp->m;
 458         traceback->c = runtime_callers(1, traceback->locbuf,
 459                 sizeof traceback->locbuf / sizeof traceback->locbuf[0], false);
 460         gp->m = holdm;
 461         runtime_gogo(traceback->gp);
 462 }
 463
 464 // Called by pthread_create to start an M.
 465 void*
 466 runtime_mstart(void *arg)
 467 {
 468         M* mp;
 469         G* gp;
 470
 471         mp = (M*)(arg);
 472         gp = mp->g0;
 473         gp->m = mp;
 474
 475         g = gp;
 476
 477         gp->entry = nil;
 478         gp->param = nil;
 479
 480         initcontext();
 481
 482         // Record top of stack for use by mcall.
 483         // Once we call schedule we're never coming back,
 484         // so other calls can reuse this stack space.
 485 #ifdef USING_SPLIT_STACK
 486         __splitstack_getcontext((void*)(&gp->stackcontext[0]));
 487 #else
 488         gp->gcinitialsp = &arg;
 489         // Setting gcstacksize to 0 is a marker meaning that gcinitialsp
 490         // is the top of the stack, not the bottom.
 491         gp->gcstacksize = 0;
 492         gp->gcnextsp = (uintptr)(&arg);
 493 #endif
 494
 495         // Save the currently active context.  This will return
 496         // multiple times via the setcontext call in mcall.
 497         getcontext(ucontext_arg(&gp->context[0]));
 498
 499         if(gp->traceback != nil) {
 500                 // Got here from getTraceback.
 501                 // I'm not sure this ever actually happens--getTraceback
 502                 // may always go to the getcontext call in mcall.
 503                 gtraceback(gp);
 504         }
 505
 506         if(gp->entry != nil) {
 507                 // Got here from mcall.
 508                 FuncVal *fv = gp->entry;
 509                 void (*pfn)(G*) = (void (*)(G*))fv->fn;
 510                 G* gp1 = (G*)gp->param;
 511                 gp->entry = nil;
 512                 gp->param = nil;
 513                 __builtin_call_with_static_chain(pfn(gp1), fv);
 514                 *(int*)0x21 = 0x21;
 515         }
 516
 517         // Initial call to getcontext--starting thread.
 518
 519 #ifdef USING_SPLIT_STACK
 520         {
 521                 int dont_block_signals = 0;
 522                 __splitstack_block_signals(&dont_block_signals, nil);
 523         }
 524 #endif
 525
 526         mstart1();
 527
 528         // mstart1 does not return, but we need a return statement
 529         // here to avoid a compiler warning.
 530         return nil;
 531 }
 532
 533 typedef struct CgoThreadStart CgoThreadStart;
 534 struct CgoThreadStart
 535 {
 536         M *m;
 537         G *g;
 538         uintptr *tls;
 539         void (*fn)(void);
 540 };
 541
 542 void setGContext(void) __asm__ (GOSYM_PREFIX "runtime.setGContext");
 543
 544 // setGContext sets up a new goroutine context for the current g.
 545 void
 546 setGContext()
 547 {
 548         int val;
 549         G *gp;
 550
 551         initcontext();
 552         gp = g;
 553         gp->entry = nil;
 554         gp->param = nil;
 555 #ifdef USING_SPLIT_STACK
 556         __splitstack_getcontext((void*)(&gp->stackcontext[0]));
 557         val = 0;
 558         __splitstack_block_signals(&val, nil);
 559 #else
 560         gp->gcinitialsp = &val;
 561         gp->gcstack = 0;
 562         gp->gcstacksize = 0;
 563         gp->gcnextsp = (uintptr)(&val);
 564 #endif
 565         getcontext(ucontext_arg(&gp->context[0]));
 566
 567         if(gp->entry != nil) {
 568                 // Got here from mcall.
 569                 FuncVal *fv = gp->entry;
 570                 void (*pfn)(G*) = (void (*)(G*))fv->fn;
 571                 G* gp1 = (G*)gp->param;
 572                 gp->entry = nil;
 573                 gp->param = nil;
 574                 __builtin_call_with_static_chain(pfn(gp1), fv);
 575                 *(int*)0x22 = 0x22;
 576         }
 577 }
 578
 579 void makeGContext(G*, byte*, uintptr)
 580         __asm__(GOSYM_PREFIX "runtime.makeGContext");
 581
 582 // makeGContext makes a new context for a g.
 583 void
 584 makeGContext(G* gp, byte* sp, uintptr spsize) {
 585         ucontext_t *uc;
 586
 587         uc = ucontext_arg(&gp->context[0]);
 588         getcontext(uc);
 589         uc->uc_stack.ss_sp = sp;
 590         uc->uc_stack.ss_size = (size_t)spsize;
 591         makecontext(uc, kickoff, 0);
 592 }
 593
 594 // The goroutine g is about to enter a system call.
 595 // Record that it's not using the cpu anymore.
 596 // This is called only from the go syscall library and cgocall,
 597 // not from the low-level system calls used by the runtime.
 598 //
 599 // Entersyscall cannot split the stack: the runtime_gosave must
 600 // make g->sched refer to the caller's stack segment, because
 601 // entersyscall is going to return immediately after.
 602
 603 void runtime_entersyscall(int32) __attribute__ ((no_split_stack));
 604 static void doentersyscall(uintptr, uintptr)
 605   __attribute__ ((no_split_stack, noinline));
 606
 607 void
 608 runtime_entersyscall(int32 dummy __attribute__ ((unused)))
 609 {
 610         // Save the registers in the g structure so that any pointers
 611         // held in registers will be seen by the garbage collector.
 612         getcontext(ucontext_arg(&g->gcregs[0]));
 613
 614         // Note that if this function does save any registers itself,
 615         // we might store the wrong value in the call to getcontext.
 616         // FIXME: This assumes that we do not need to save any
 617         // callee-saved registers to access the TLS variable g.  We
 618         // don't want to put the ucontext_t on the stack because it is
 619         // large and we can not split the stack here.
 620         doentersyscall((uintptr)runtime_getcallerpc(&dummy),
 621                        (uintptr)runtime_getcallersp(&dummy));
 622 }
 623
 624 static void
 625 doentersyscall(uintptr pc, uintptr sp)
 626 {
 627         // Leave SP around for GC and traceback.
 628 #ifdef USING_SPLIT_STACK
 629         {
 630           size_t gcstacksize;
 631           g->gcstack = (uintptr)(__splitstack_find(nil, nil, &gcstacksize,
 632                                                    (void**)(&g->gcnextsegment),
 633                                                    (void**)(&g->gcnextsp),
 634                                                    &g->gcinitialsp));
 635           g->gcstacksize = (uintptr)gcstacksize;
 636         }
 637 #else
 638         {
 639                 void *v;
 640
 641                 g->gcnextsp = (uintptr)(&v);
 642         }
 643 #endif
 644
 645         reentersyscall(pc, sp);
 646 }
 647
 648 static void doentersyscallblock(uintptr, uintptr)
 649   __attribute__ ((no_split_stack, noinline));
 650
 651 // The same as runtime_entersyscall(), but with a hint that the syscall is blocking.
 652 void
 653 runtime_entersyscallblock(int32 dummy __attribute__ ((unused)))
 654 {
 655         // Save the registers in the g structure so that any pointers
 656         // held in registers will be seen by the garbage collector.
 657         getcontext(ucontext_arg(&g->gcregs[0]));
 658
 659         // See comment in runtime_entersyscall.
 660         doentersyscallblock((uintptr)runtime_getcallerpc(&dummy),
 661                             (uintptr)runtime_getcallersp(&dummy));
 662 }
 663
 664 static void
 665 doentersyscallblock(uintptr pc, uintptr sp)
 666 {
 667         // Leave SP around for GC and traceback.
 668 #ifdef USING_SPLIT_STACK
 669         {
 670           size_t gcstacksize;
 671           g->gcstack = (uintptr)(__splitstack_find(nil, nil, &gcstacksize,
 672                                                    (void**)(&g->gcnextsegment),
 673                                                    (void**)(&g->gcnextsp),
 674                                                    &g->gcinitialsp));
 675           g->gcstacksize = (uintptr)gcstacksize;
 676         }
 677 #else
 678         {
 679                 void *v;
 680
 681                 g->gcnextsp = (uintptr)(&v);
 682         }
 683 #endif
 684
 685         reentersyscallblock(pc, sp);
 686 }
 687
 688 // Allocate a new g, with a stack big enough for stacksize bytes.
 689 G*
 690 runtime_malg(bool allocatestack, bool signalstack, byte** ret_stack, uintptr* ret_stacksize)
 691 {
 692         uintptr stacksize;
 693         G *newg;
 694         byte* unused_stack;
 695         uintptr unused_stacksize;
 696 #if USING_SPLIT_STACK
 697         int dont_block_signals = 0;
 698         size_t ss_stacksize;
 699 #endif
 700
 701         if (ret_stack == nil) {
 702                 ret_stack = &unused_stack;
 703         }
 704         if (ret_stacksize == nil) {
 705                 ret_stacksize = &unused_stacksize;
 706         }
 707         newg = allocg();
 708         if(allocatestack) {
 709                 stacksize = StackMin;
 710                 if(signalstack) {
 711                         stacksize = 32 * 1024; // OS X wants >= 8K, GNU/Linux >= 2K
 712 #ifdef SIGSTKSZ
 713                         if(stacksize < SIGSTKSZ)
 714                                 stacksize = SIGSTKSZ;
 715 #endif
 716                 }
 717
 718 #if USING_SPLIT_STACK
 719                 *ret_stack = __splitstack_makecontext(stacksize,
 720                                                       (void*)(&newg->stackcontext[0]),
 721                                                       &ss_stacksize);
 722                 *ret_stacksize = (uintptr)ss_stacksize;
 723                 __splitstack_block_signals_context((void*)(&newg->stackcontext[0]),
 724                                                    &dont_block_signals, nil);
 725 #else
 726                 // In 64-bit mode, the maximum Go allocation space is
 727                 // 128G.  Our stack size is 4M, which only permits 32K
 728                 // goroutines.  In order to not limit ourselves,
 729                 // allocate the stacks out of separate memory.  In
 730                 // 32-bit mode, the Go allocation space is all of
 731                 // memory anyhow.
 732                 if(sizeof(void*) == 8) {
 733                         void *p = runtime_sysAlloc(stacksize, &getMemstats()->stacks_sys);
 734                         if(p == nil)
 735                                 runtime_throw("runtime: cannot allocate memory for goroutine stack");
 736                         *ret_stack = (byte*)p;
 737                 } else {
 738                         *ret_stack = runtime_mallocgc(stacksize, nil, false);
 739                         runtime_xadd(&runtime_stacks_sys, stacksize);
 740                 }
 741                 *ret_stacksize = (uintptr)stacksize;
 742                 newg->gcinitialsp = *ret_stack;
 743                 newg->gcstacksize = (uintptr)stacksize;
 744 #endif
 745         }
 746         return newg;
 747 }
 748
 749 void resetNewG(G*, void **, uintptr*)
 750   __asm__(GOSYM_PREFIX "runtime.resetNewG");
 751
 752 // Reset stack information for g pulled out of the cache to start a
 753 // new goroutine.
 754 void
 755 resetNewG(G *newg, void **sp, uintptr *spsize)
 756 {
 757 #ifdef USING_SPLIT_STACK
 758   int dont_block_signals = 0;
 759   size_t ss_spsize;
 760
 761   *sp = __splitstack_resetcontext((void*)(&newg->stackcontext[0]), &ss_spsize);
 762   *spsize = ss_spsize;
 763   __splitstack_block_signals_context((void*)(&newg->stackcontext[0]),
 764                                      &dont_block_signals, nil);
 765 #else
 766   *sp = newg->gcinitialsp;
 767   *spsize = newg->gcstacksize;
 768   if(*spsize == 0)
 769     runtime_throw("bad spsize in resetNewG");
 770   newg->gcnextsp = (uintptr)(*sp);
 771 #endif
 772 }
 773
 774 // Return whether we are waiting for a GC.  This gc toolchain uses
 775 // preemption instead.
 776 bool
 777 runtime_gcwaiting(void)
 778 {
 779         return runtime_sched->gcwaiting;
 780 }