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coroutine: adding sigaltstack method (.c source)

This file is based in both coroutine-ucontext.c and
pth_mctx.c (from the GNU Portable Threads library).

The mechanism used to change stacks is the sigaltstack
function (variant 2 of the pth library).

v2: Some corrections. Moving global variables into
thread storage (CoroutineThreadState).

Signed-off-by: Alex Barcelo <abarcelo@ac.upc.edu>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Alex Barcelo Kevin Wolf
Alex Barcelo authored and Kevin Wolf committed Feb 28, 2012
1 parent 250196f commit 3194c8ceeba06c3b54621ea7afd6879bc50c2d1d
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  1. +334 −0 coroutine-sigaltstack.c
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+/*
+ * sigaltstack coroutine initialization code
+ *
+ * Copyright (C) 2006 Anthony Liguori <anthony@codemonkey.ws>
+ * Copyright (C) 2011 Kevin Wolf <kwolf@redhat.com>
+ * Copyright (C) 2012 Alex Barcelo <abarcelo@ac.upc.edu>
+** This file is partly based on pth_mctx.c, from the GNU Portable Threads
+** Copyright (c) 1999-2006 Ralf S. Engelschall <rse@engelschall.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+/* XXX Is there a nicer way to disable glibc's stack check for longjmp? */
+#ifdef _FORTIFY_SOURCE
+#undef _FORTIFY_SOURCE
+#endif
+#include <stdlib.h>
+#include <setjmp.h>
+#include <stdint.h>
+#include <pthread.h>
+#include <signal.h>
+#include "qemu-common.h"
+#include "qemu-coroutine-int.h"
+
+enum {
+ /* Maximum free pool size prevents holding too many freed coroutines */
+ POOL_MAX_SIZE = 64,
+};
+
+/** Free list to speed up creation */
+static QSLIST_HEAD(, Coroutine) pool = QSLIST_HEAD_INITIALIZER(pool);
+static unsigned int pool_size;
+
+typedef struct {
+ Coroutine base;
+ void *stack;
+ jmp_buf env;
+} CoroutineUContext;
+
+/**
+ * Per-thread coroutine bookkeeping
+ */
+typedef struct {
+ /** Currently executing coroutine */
+ Coroutine *current;
+
+ /** The default coroutine */
+ CoroutineUContext leader;
+
+ /** Information for the signal handler (trampoline) */
+ jmp_buf tr_reenter;
+ volatile sig_atomic_t tr_called;
+ void *tr_handler;
+} CoroutineThreadState;
+
+static pthread_key_t thread_state_key;
+
+static CoroutineThreadState *coroutine_get_thread_state(void)
+{
+ CoroutineThreadState *s = pthread_getspecific(thread_state_key);
+
+ if (!s) {
+ s = g_malloc0(sizeof(*s));
+ s->current = &s->leader.base;
+ pthread_setspecific(thread_state_key, s);
+ }
+ return s;
+}
+
+static void qemu_coroutine_thread_cleanup(void *opaque)
+{
+ CoroutineThreadState *s = opaque;
+
+ g_free(s);
+}
+
+static void __attribute__((destructor)) coroutine_cleanup(void)
+{
+ Coroutine *co;
+ Coroutine *tmp;
+
+ QSLIST_FOREACH_SAFE(co, &pool, pool_next, tmp) {
+ g_free(DO_UPCAST(CoroutineUContext, base, co)->stack);
+ g_free(co);
+ }
+}
+
+static void __attribute__((constructor)) coroutine_init(void)
+{
+ int ret;
+
+ ret = pthread_key_create(&thread_state_key, qemu_coroutine_thread_cleanup);
+ if (ret != 0) {
+ fprintf(stderr, "unable to create leader key: %s\n", strerror(errno));
+ abort();
+ }
+}
+
+/* "boot" function
+ * This is what starts the coroutine, is called from the trampoline
+ * (from the signal handler when it is not signal handling, read ahead
+ * for more information).
+ */
+static void coroutine_bootstrap(CoroutineUContext *self, Coroutine *co)
+{
+ /* Initialize longjmp environment and switch back the caller */
+ if (!setjmp(self->env)) {
+ longjmp(*(jmp_buf *)co->entry_arg, 1);
+ }
+
+ while (true) {
+ co->entry(co->entry_arg);
+ qemu_coroutine_switch(co, co->caller, COROUTINE_TERMINATE);
+ }
+}
+
+/*
+ * This is used as the signal handler. This is called with the brand new stack
+ * (thanks to sigaltstack). We have to return, given that this is a signal
+ * handler and the sigmask and some other things are changed.
+ */
+static void coroutine_trampoline(int signal)
+{
+ CoroutineUContext *self;
+ Coroutine *co;
+ CoroutineThreadState *coTS;
+
+ /* Get the thread specific information */
+ coTS = coroutine_get_thread_state();
+ self = coTS->tr_handler;
+ coTS->tr_called = 1;
+ co = &self->base;
+
+ /*
+ * Here we have to do a bit of a ping pong between the caller, given that
+ * this is a signal handler and we have to do a return "soon". Then the
+ * caller can reestablish everything and do a longjmp here again.
+ */
+ if (!setjmp(coTS->tr_reenter)) {
+ return;
+ }
+
+ /*
+ * Ok, the caller has longjmp'ed back to us, so now prepare
+ * us for the real machine state switching. We have to jump
+ * into another function here to get a new stack context for
+ * the auto variables (which have to be auto-variables
+ * because the start of the thread happens later). Else with
+ * PIC (i.e. Position Independent Code which is used when PTH
+ * is built as a shared library) most platforms would
+ * horrible core dump as experience showed.
+ */
+ coroutine_bootstrap(self, co);
+}
+
+static Coroutine *coroutine_new(void)
+{
+ const size_t stack_size = 1 << 20;
+ CoroutineUContext *co;
+ CoroutineThreadState *coTS;
+ struct sigaction sa;
+ struct sigaction osa;
+ struct sigaltstack ss;
+ struct sigaltstack oss;
+ sigset_t sigs;
+ sigset_t osigs;
+ jmp_buf old_env;
+
+ /* The way to manipulate stack is with the sigaltstack function. We
+ * prepare a stack, with it delivering a signal to ourselves and then
+ * put setjmp/longjmp where needed.
+ * This has been done keeping coroutine-ucontext as a model and with the
+ * pth ideas (GNU Portable Threads). See coroutine-ucontext for the basics
+ * of the coroutines and see pth_mctx.c (from the pth project) for the
+ * sigaltstack way of manipulating stacks.
+ */
+
+ co = g_malloc0(sizeof(*co));
+ co->stack = g_malloc(stack_size);
+ co->base.entry_arg = &old_env; /* stash away our jmp_buf */
+
+ coTS = coroutine_get_thread_state();
+ coTS->tr_handler = co;
+
+ /*
+ * Preserve the SIGUSR2 signal state, block SIGUSR2,
+ * and establish our signal handler. The signal will
+ * later transfer control onto the signal stack.
+ */
+ sigemptyset(&sigs);
+ sigaddset(&sigs, SIGUSR2);
+ pthread_sigmask(SIG_BLOCK, &sigs, &osigs);
+ sa.sa_handler = coroutine_trampoline;
+ sigfillset(&sa.sa_mask);
+ sa.sa_flags = SA_ONSTACK;
+ if (sigaction(SIGUSR2, &sa, &osa) != 0) {
+ abort();
+ }
+
+ /*
+ * Set the new stack.
+ */
+ ss.ss_sp = co->stack;
+ ss.ss_size = stack_size;
+ ss.ss_flags = 0;
+ if (sigaltstack(&ss, &oss) < 0) {
+ abort();
+ }
+
+ /*
+ * Now transfer control onto the signal stack and set it up.
+ * It will return immediately via "return" after the setjmp()
+ * was performed. Be careful here with race conditions. The
+ * signal can be delivered the first time sigsuspend() is
+ * called.
+ */
+ coTS->tr_called = 0;
+ kill(getpid(), SIGUSR2);
+ sigfillset(&sigs);
+ sigdelset(&sigs, SIGUSR2);
+ while (!coTS->tr_called) {
+ sigsuspend(&sigs);
+ }
+
+ /*
+ * Inform the system that we are back off the signal stack by
+ * removing the alternative signal stack. Be careful here: It
+ * first has to be disabled, before it can be removed.
+ */
+ sigaltstack(NULL, &ss);
+ ss.ss_flags = SS_DISABLE;
+ if (sigaltstack(&ss, NULL) < 0) {
+ abort();
+ }
+ sigaltstack(NULL, &ss);
+ if (!(oss.ss_flags & SS_DISABLE)) {
+ sigaltstack(&oss, NULL);
+ }
+
+ /*
+ * Restore the old SIGUSR2 signal handler and mask
+ */
+ sigaction(SIGUSR2, &osa, NULL);
+ pthread_sigmask(SIG_SETMASK, &osigs, NULL);
+
+ /*
+ * Now enter the trampoline again, but this time not as a signal
+ * handler. Instead we jump into it directly. The functionally
+ * redundant ping-pong pointer arithmentic is neccessary to avoid
+ * type-conversion warnings related to the `volatile' qualifier and
+ * the fact that `jmp_buf' usually is an array type.
+ */
+ if (!setjmp(old_env)) {
+ longjmp(coTS->tr_reenter, 1);
+ }
+
+ /*
+ * Ok, we returned again, so now we're finished
+ */
+
+ return &co->base;
+}
+
+Coroutine *qemu_coroutine_new(void)
+{
+ Coroutine *co;
+
+ co = QSLIST_FIRST(&pool);
+ if (co) {
+ QSLIST_REMOVE_HEAD(&pool, pool_next);
+ pool_size--;
+ } else {
+ co = coroutine_new();
+ }
+ return co;
+}
+
+void qemu_coroutine_delete(Coroutine *co_)
+{
+ CoroutineUContext *co = DO_UPCAST(CoroutineUContext, base, co_);
+
+ if (pool_size < POOL_MAX_SIZE) {
+ QSLIST_INSERT_HEAD(&pool, &co->base, pool_next);
+ co->base.caller = NULL;
+ pool_size++;
+ return;
+ }
+
+ g_free(co->stack);
+ g_free(co);
+}
+
+CoroutineAction qemu_coroutine_switch(Coroutine *from_, Coroutine *to_,
+ CoroutineAction action)
+{
+ CoroutineUContext *from = DO_UPCAST(CoroutineUContext, base, from_);
+ CoroutineUContext *to = DO_UPCAST(CoroutineUContext, base, to_);
+ CoroutineThreadState *s = coroutine_get_thread_state();
+ int ret;
+
+ s->current = to_;
+
+ ret = setjmp(from->env);
+ if (ret == 0) {
+ longjmp(to->env, action);
+ }
+ return ret;
+}
+
+Coroutine *qemu_coroutine_self(void)
+{
+ CoroutineThreadState *s = coroutine_get_thread_state();
+
+ return s->current;
+}
+
+bool qemu_in_coroutine(void)
+{
+ CoroutineThreadState *s = pthread_getspecific(thread_state_key);
+
+ return s && s->current->caller;
+}
+

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