compat_arch_x86.c

   1 /*
   2  * compat_arch_x86.c
   3  *
   4  * Userspace RCU library - x86 compatibility checks
   5  *
   6  * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
   7  *
   8  * This library is free software; you can redistribute it and/or
   9  * modify it under the terms of the GNU Lesser General Public
  10  * License as published by the Free Software Foundation; either
  11  * version 2.1 of the License, or (at your option) any later version.
  12  *
  13  * This library is distributed in the hope that it will be useful,
  14  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  15  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  16  * Lesser General Public License for more details.
  17  *
  18  * You should have received a copy of the GNU Lesser General Public
  19  * License along with this library; if not, write to the Free Software
  20  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  21  */
  22
  23 #include <stdio.h>
  24 #include <pthread.h>
  25 #include <signal.h>
  26 #include <assert.h>
  27 #include <urcu/uatomic_arch.h>
  28
  29 /*
  30  * It does not really matter if the constructor is called before using
  31  * the library, as long as the caller checks if __urcu_cas_avail < 0 and calls
  32  * compat_arch_init() explicitely if needed.
  33  */
  34 int __attribute__((constructor)) __urcu_cas_init(void);
  35
  36 static pthread_mutex_t compat_mutex = PTHREAD_MUTEX_INITIALIZER;
  37
  38 /*
  39  * -1: unknown
  40  *  1: available
  41  *  0: unavailable
  42  */
  43 int __urcu_cas_avail = -1;
  44
  45 /*
  46  * Imported from glibc 2.3.5. linuxthreads/sysdeps/i386/pt-machine.h.
  47  */
  48
  49 int get_eflags (void)
  50 {
  51         int res;
  52         __asm__ __volatile__ ("pushfl; popl %0" : "=r" (res) : );
  53         return res;
  54 }
  55
  56 void set_eflags (int newflags)
  57 {
  58         __asm__ __volatile__ ("pushl %0; popfl" : : "r" (newflags) : "cc");
  59 }
  60
  61 int compare_and_swap_is_available (void)
  62 {
  63         int oldflags = get_eflags ();
  64         int changed;
  65         /* Flip AC bit in EFLAGS.  */
  66         set_eflags (oldflags ^ 0x40000);
  67         /* See if bit changed.  */
  68         changed = (get_eflags () ^ oldflags) & 0x40000;
  69         /* Restore EFLAGS.  */
  70         set_eflags (oldflags);
  71         /* If the AC flag did not change, it's a 386 and it lacks cmpxchg.
  72         Otherwise, it's a 486 or above and it has cmpxchg.  */
  73         return changed != 0;
  74 }
  75
  76 unsigned long _compat_uatomic_cmpxchg(void *addr, unsigned long old,
  77                               unsigned long _new, int len)
  78 {
  79         sigset_t newmask, oldmask;
  80         int ret;
  81
  82         /* Disable signals */
  83         ret = sigemptyset(&newmask);
  84         assert(!ret);
  85         ret = pthread_sigmask(SIG_SETMASK, &newmask, &oldmask);
  86         assert(!ret);
  87         ret = pthread_mutex_lock(&compat_mutex);
  88         assert(!ret);
  89
  90         switch (len) {
  91         case 1:
  92         {
  93                 unsigned char result = *(unsigned char *)addr;
  94                 if (result == old)
  95                         *(unsigned char *)addr = (unsigned char)_new;
  96                 return result;
  97         }
  98         case 2:
  99         {
 100                 unsigned short result = *(unsigned short *)addr;
 101                 if (result == old)
 102                         *(unsigned short *)addr = (unsigned short)_new;
 103                 return result;
 104         }
 105         case 4:
 106         {
 107                 unsigned int result = *(unsigned int *)addr;
 108                 if (result == old)
 109                         *(unsigned int *)addr = (unsigned int)_new;
 110                 return result;
 111         }
 112         }
 113         /* generate an illegal instruction. Cannot catch this with linker tricks
 114          * when optimizations are disabled. */
 115         __asm__ __volatile__("ud2");
 116         return 0;
 117
 118         ret = pthread_mutex_unlock(&compat_mutex);
 119         assert(!ret);
 120         ret = pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
 121         assert(!ret);
 122 }
 123
 124 int __urcu_cas_init(void)
 125 {
 126         if (__urcu_cas_avail < 0)
 127                 __urcu_cas_avail = compare_and_swap_is_available();
 128         return __urcu_cas_avail;
 129 }