coroutine-ucontext.c

   1 /*
   2  * ucontext coroutine initialization code
   3  *
   4  * Copyright (C) 2006  Anthony Liguori <anthony@codemonkey.ws>
   5  * Copyright (C) 2011  Kevin Wolf <kwolf@redhat.com>
   6  *
   7  * This library is free software; you can redistribute it and/or
   8  * modify it under the terms of the GNU Lesser General Public
   9  * License as published by the Free Software Foundation; either
  10  * version 2.0 of the License, or (at your option) any later version.
  11  *
  12  * This library is distributed in the hope that it will be useful,
  13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  15  * Lesser General Public License for more details.
  16  *
  17  * You should have received a copy of the GNU Lesser General Public
  18  * License along with this library; if not, see <http://www.gnu.org/licenses/>.
  19  */
  20
  21 /* XXX Is there a nicer way to disable glibc's stack check for longjmp? */
  22 #ifdef _FORTIFY_SOURCE
  23 #undef _FORTIFY_SOURCE
  24 #endif
  25 #include <stdlib.h>
  26 #include <setjmp.h>
  27 #include <stdint.h>
  28 #include <ucontext.h>
  29 #include "qemu-common.h"
  30 #include "block/coroutine_int.h"
  31
  32 #ifdef CONFIG_VALGRIND_H
  33 #include <valgrind/valgrind.h>
  34 #endif
  35
  36 typedef struct {
  37     Coroutine base;
  38     void *stack;
  39     sigjmp_buf env;
  40
  41 #ifdef CONFIG_VALGRIND_H
  42     unsigned int valgrind_stack_id;
  43 #endif
  44
  45 } CoroutineUContext;
  46
  47 /**
  48  * Per-thread coroutine bookkeeping
  49  */
  50 static __thread CoroutineUContext leader;
  51 static __thread Coroutine *current;
  52
  53 /*
  54  * va_args to makecontext() must be type 'int', so passing
  55  * the pointer we need may require several int args. This
  56  * union is a quick hack to let us do that
  57  */
  58 union cc_arg {
  59     void *p;
  60     int i[2];
  61 };
  62
  63 static void coroutine_trampoline(int i0, int i1)
  64 {
  65     union cc_arg arg;
  66     CoroutineUContext *self;
  67     Coroutine *co;
  68
  69     arg.i[0] = i0;
  70     arg.i[1] = i1;
  71     self = arg.p;
  72     co = &self->base;
  73
  74     /* Initialize longjmp environment and switch back the caller */
  75     if (!sigsetjmp(self->env, 0)) {
  76         siglongjmp(*(sigjmp_buf *)co->entry_arg, 1);
  77     }
  78
  79     while (true) {
  80         co->entry(co->entry_arg);
  81         qemu_coroutine_switch(co, co->caller, COROUTINE_TERMINATE);
  82     }
  83 }
  84
  85 Coroutine *qemu_coroutine_new(void)
  86 {
  87     const size_t stack_size = 1 << 20;
  88     CoroutineUContext *co;
  89     ucontext_t old_uc, uc;
  90     sigjmp_buf old_env;
  91     union cc_arg arg = {0};
  92
  93     /* The ucontext functions preserve signal masks which incurs a
  94      * system call overhead.  sigsetjmp(buf, 0)/siglongjmp() does not
  95      * preserve signal masks but only works on the current stack.
  96      * Since we need a way to create and switch to a new stack, use
  97      * the ucontext functions for that but sigsetjmp()/siglongjmp() for
  98      * everything else.
  99      */
 100
 101     if (getcontext(&uc) == -1) {
 102         abort();
 103     }
 104
 105     co = g_malloc0(sizeof(*co));
 106     co->stack = g_malloc(stack_size);
 107     co->base.entry_arg = &old_env; /* stash away our jmp_buf */
 108
 109     uc.uc_link = &old_uc;
 110     uc.uc_stack.ss_sp = co->stack;
 111     uc.uc_stack.ss_size = stack_size;
 112     uc.uc_stack.ss_flags = 0;
 113
 114 #ifdef CONFIG_VALGRIND_H
 115     co->valgrind_stack_id =
 116         VALGRIND_STACK_REGISTER(co->stack, co->stack + stack_size);
 117 #endif
 118
 119     arg.p = co;
 120
 121     makecontext(&uc, (void (*)(void))coroutine_trampoline,
 122                 2, arg.i[0], arg.i[1]);
 123
 124     /* swapcontext() in, siglongjmp() back out */
 125     if (!sigsetjmp(old_env, 0)) {
 126         swapcontext(&old_uc, &uc);
 127     }
 128     return &co->base;
 129 }
 130
 131 #ifdef CONFIG_VALGRIND_H
 132 #ifdef CONFIG_PRAGMA_DIAGNOSTIC_AVAILABLE
 133 /* Work around an unused variable in the valgrind.h macro... */
 134 #pragma GCC diagnostic push
 135 #pragma GCC diagnostic ignored "-Wunused-but-set-variable"
 136 #endif
 137 static inline void valgrind_stack_deregister(CoroutineUContext *co)
 138 {
 139     VALGRIND_STACK_DEREGISTER(co->valgrind_stack_id);
 140 }
 141 #ifdef CONFIG_PRAGMA_DIAGNOSTIC_AVAILABLE
 142 #pragma GCC diagnostic pop
 143 #endif
 144 #endif
 145
 146 void qemu_coroutine_delete(Coroutine *co_)
 147 {
 148     CoroutineUContext *co = DO_UPCAST(CoroutineUContext, base, co_);
 149
 150 #ifdef CONFIG_VALGRIND_H
 151     valgrind_stack_deregister(co);
 152 #endif
 153
 154     g_free(co->stack);
 155     g_free(co);
 156 }
 157
 158 /* This function is marked noinline to prevent GCC from inlining it
 159  * into coroutine_trampoline(). If we allow it to do that then it
 160  * hoists the code to get the address of the TLS variable "current"
 161  * out of the while() loop. This is an invalid transformation because
 162  * the sigsetjmp() call may be called when running thread A but
 163  * return in thread B, and so we might be in a different thread
 164  * context each time round the loop.
 165  */
 166 CoroutineAction __attribute__((noinline))
 167 qemu_coroutine_switch(Coroutine *from_, Coroutine *to_,
 168                       CoroutineAction action)
 169 {
 170     CoroutineUContext *from = DO_UPCAST(CoroutineUContext, base, from_);
 171     CoroutineUContext *to = DO_UPCAST(CoroutineUContext, base, to_);
 172     int ret;
 173
 174     current = to_;
 175
 176     ret = sigsetjmp(from->env, 0);
 177     if (ret == 0) {
 178         siglongjmp(to->env, action);
 179     }
 180     return ret;
 181 }
 182
 183 Coroutine *qemu_coroutine_self(void)
 184 {
 185     if (!current) {
 186         current = &leader.base;
 187     }
 188     return current;
 189 }
 190
 191 bool qemu_in_coroutine(void)
 192 {
 193     return current && current->caller;
 194 }