coroutine-sigaltstack.c

   1 /*
   2  * sigaltstack coroutine initialization code
   3  *
   4  * Copyright (C) 2006  Anthony Liguori <anthony@codemonkey.ws>
   5  * Copyright (C) 2011  Kevin Wolf <kwolf@redhat.com>
   6  * Copyright (C) 2012  Alex Barcelo <abarcelo@ac.upc.edu>
   7 ** This file is partly based on pth_mctx.c, from the GNU Portable Threads
   8 **  Copyright (c) 1999-2006 Ralf S. Engelschall <rse@engelschall.com>
   9  *
  10  * This library is free software; you can redistribute it and/or
  11  * modify it under the terms of the GNU Lesser General Public
  12  * License as published by the Free Software Foundation; either
  13  * version 2.1 of the License, or (at your option) any later version.
  14  *
  15  * This library is distributed in the hope that it will be useful,
  16  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  17  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  18  * Lesser General Public License for more details.
  19  *
  20  * You should have received a copy of the GNU Lesser General Public
  21  * License along with this library; if not, see <http://www.gnu.org/licenses/>.
  22  */
  23
  24 /* XXX Is there a nicer way to disable glibc's stack check for longjmp? */
  25 #ifdef _FORTIFY_SOURCE
  26 #undef _FORTIFY_SOURCE
  27 #endif
  28 #include <stdlib.h>
  29 #include <setjmp.h>
  30 #include <stdint.h>
  31 #include <pthread.h>
  32 #include <signal.h>
  33 #include "qemu-common.h"
  34 #include "block/coroutine_int.h"
  35
  36 typedef struct {
  37     Coroutine base;
  38     void *stack;
  39     sigjmp_buf env;
  40 } CoroutineUContext;
  41
  42 /**
  43  * Per-thread coroutine bookkeeping
  44  */
  45 typedef struct {
  46     /** Currently executing coroutine */
  47     Coroutine *current;
  48
  49     /** The default coroutine */
  50     CoroutineUContext leader;
  51
  52     /** Information for the signal handler (trampoline) */
  53     sigjmp_buf tr_reenter;
  54     volatile sig_atomic_t tr_called;
  55     void *tr_handler;
  56 } CoroutineThreadState;
  57
  58 static pthread_key_t thread_state_key;
  59
  60 static CoroutineThreadState *coroutine_get_thread_state(void)
  61 {
  62     CoroutineThreadState *s = pthread_getspecific(thread_state_key);
  63
  64     if (!s) {
  65         s = g_malloc0(sizeof(*s));
  66         s->current = &s->leader.base;
  67         pthread_setspecific(thread_state_key, s);
  68     }
  69     return s;
  70 }
  71
  72 static void qemu_coroutine_thread_cleanup(void *opaque)
  73 {
  74     CoroutineThreadState *s = opaque;
  75
  76     g_free(s);
  77 }
  78
  79 static void __attribute__((constructor)) coroutine_init(void)
  80 {
  81     int ret;
  82
  83     ret = pthread_key_create(&thread_state_key, qemu_coroutine_thread_cleanup);
  84     if (ret != 0) {
  85         fprintf(stderr, "unable to create leader key: %s\n", strerror(errno));
  86         abort();
  87     }
  88 }
  89
  90 /* "boot" function
  91  * This is what starts the coroutine, is called from the trampoline
  92  * (from the signal handler when it is not signal handling, read ahead
  93  * for more information).
  94  */
  95 static void coroutine_bootstrap(CoroutineUContext *self, Coroutine *co)
  96 {
  97     /* Initialize longjmp environment and switch back the caller */
  98     if (!sigsetjmp(self->env, 0)) {
  99         siglongjmp(*(sigjmp_buf *)co->entry_arg, 1);
 100     }
 101
 102     while (true) {
 103         co->entry(co->entry_arg);
 104         qemu_coroutine_switch(co, co->caller, COROUTINE_TERMINATE);
 105     }
 106 }
 107
 108 /*
 109  * This is used as the signal handler. This is called with the brand new stack
 110  * (thanks to sigaltstack). We have to return, given that this is a signal
 111  * handler and the sigmask and some other things are changed.
 112  */
 113 static void coroutine_trampoline(int signal)
 114 {
 115     CoroutineUContext *self;
 116     Coroutine *co;
 117     CoroutineThreadState *coTS;
 118
 119     /* Get the thread specific information */
 120     coTS = coroutine_get_thread_state();
 121     self = coTS->tr_handler;
 122     coTS->tr_called = 1;
 123     co = &self->base;
 124
 125     /*
 126      * Here we have to do a bit of a ping pong between the caller, given that
 127      * this is a signal handler and we have to do a return "soon". Then the
 128      * caller can reestablish everything and do a siglongjmp here again.
 129      */
 130     if (!sigsetjmp(coTS->tr_reenter, 0)) {
 131         return;
 132     }
 133
 134     /*
 135      * Ok, the caller has siglongjmp'ed back to us, so now prepare
 136      * us for the real machine state switching. We have to jump
 137      * into another function here to get a new stack context for
 138      * the auto variables (which have to be auto-variables
 139      * because the start of the thread happens later). Else with
 140      * PIC (i.e. Position Independent Code which is used when PTH
 141      * is built as a shared library) most platforms would
 142      * horrible core dump as experience showed.
 143      */
 144     coroutine_bootstrap(self, co);
 145 }
 146
 147 Coroutine *qemu_coroutine_new(void)
 148 {
 149     const size_t stack_size = 1 << 20;
 150     CoroutineUContext *co;
 151     CoroutineThreadState *coTS;
 152     struct sigaction sa;
 153     struct sigaction osa;
 154     stack_t ss;
 155     stack_t oss;
 156     sigset_t sigs;
 157     sigset_t osigs;
 158     jmp_buf old_env;
 159
 160     /* The way to manipulate stack is with the sigaltstack function. We
 161      * prepare a stack, with it delivering a signal to ourselves and then
 162      * put sigsetjmp/siglongjmp where needed.
 163      * This has been done keeping coroutine-ucontext as a model and with the
 164      * pth ideas (GNU Portable Threads). See coroutine-ucontext for the basics
 165      * of the coroutines and see pth_mctx.c (from the pth project) for the
 166      * sigaltstack way of manipulating stacks.
 167      */
 168
 169     co = g_malloc0(sizeof(*co));
 170     co->stack = g_malloc(stack_size);
 171     co->base.entry_arg = &old_env; /* stash away our jmp_buf */
 172
 173     coTS = coroutine_get_thread_state();
 174     coTS->tr_handler = co;
 175
 176     /*
 177      * Preserve the SIGUSR2 signal state, block SIGUSR2,
 178      * and establish our signal handler. The signal will
 179      * later transfer control onto the signal stack.
 180      */
 181     sigemptyset(&sigs);
 182     sigaddset(&sigs, SIGUSR2);
 183     pthread_sigmask(SIG_BLOCK, &sigs, &osigs);
 184     sa.sa_handler = coroutine_trampoline;
 185     sigfillset(&sa.sa_mask);
 186     sa.sa_flags = SA_ONSTACK;
 187     if (sigaction(SIGUSR2, &sa, &osa) != 0) {
 188         abort();
 189     }
 190
 191     /*
 192      * Set the new stack.
 193      */
 194     ss.ss_sp = co->stack;
 195     ss.ss_size = stack_size;
 196     ss.ss_flags = 0;
 197     if (sigaltstack(&ss, &oss) < 0) {
 198         abort();
 199     }
 200
 201     /*
 202      * Now transfer control onto the signal stack and set it up.
 203      * It will return immediately via "return" after the sigsetjmp()
 204      * was performed. Be careful here with race conditions.  The
 205      * signal can be delivered the first time sigsuspend() is
 206      * called.
 207      */
 208     coTS->tr_called = 0;
 209     pthread_kill(pthread_self(), SIGUSR2);
 210     sigfillset(&sigs);
 211     sigdelset(&sigs, SIGUSR2);
 212     while (!coTS->tr_called) {
 213         sigsuspend(&sigs);
 214     }
 215
 216     /*
 217      * Inform the system that we are back off the signal stack by
 218      * removing the alternative signal stack. Be careful here: It
 219      * first has to be disabled, before it can be removed.
 220      */
 221     sigaltstack(NULL, &ss);
 222     ss.ss_flags = SS_DISABLE;
 223     if (sigaltstack(&ss, NULL) < 0) {
 224         abort();
 225     }
 226     sigaltstack(NULL, &ss);
 227     if (!(oss.ss_flags & SS_DISABLE)) {
 228         sigaltstack(&oss, NULL);
 229     }
 230
 231     /*
 232      * Restore the old SIGUSR2 signal handler and mask
 233      */
 234     sigaction(SIGUSR2, &osa, NULL);
 235     pthread_sigmask(SIG_SETMASK, &osigs, NULL);
 236
 237     /*
 238      * Now enter the trampoline again, but this time not as a signal
 239      * handler. Instead we jump into it directly. The functionally
 240      * redundant ping-pong pointer arithmetic is necessary to avoid
 241      * type-conversion warnings related to the `volatile' qualifier and
 242      * the fact that `jmp_buf' usually is an array type.
 243      */
 244     if (!sigsetjmp(old_env, 0)) {
 245         siglongjmp(coTS->tr_reenter, 1);
 246     }
 247
 248     /*
 249      * Ok, we returned again, so now we're finished
 250      */
 251
 252     return &co->base;
 253 }
 254
 255 void qemu_coroutine_delete(Coroutine *co_)
 256 {
 257     CoroutineUContext *co = DO_UPCAST(CoroutineUContext, base, co_);
 258
 259     g_free(co->stack);
 260     g_free(co);
 261 }
 262
 263 CoroutineAction qemu_coroutine_switch(Coroutine *from_, Coroutine *to_,
 264                                       CoroutineAction action)
 265 {
 266     CoroutineUContext *from = DO_UPCAST(CoroutineUContext, base, from_);
 267     CoroutineUContext *to = DO_UPCAST(CoroutineUContext, base, to_);
 268     CoroutineThreadState *s = coroutine_get_thread_state();
 269     int ret;
 270
 271     s->current = to_;
 272
 273     ret = sigsetjmp(from->env, 0);
 274     if (ret == 0) {
 275         siglongjmp(to->env, action);
 276     }
 277     return ret;
 278 }
 279
 280 Coroutine *qemu_coroutine_self(void)
 281 {
 282     CoroutineThreadState *s = coroutine_get_thread_state();
 283
 284     return s->current;
 285 }
 286
 287 bool qemu_in_coroutine(void)
 288 {
 289     CoroutineThreadState *s = pthread_getspecific(thread_state_key);
 290
 291     return s && s->current->caller;
 292 }
 293