1 | #include "pthread_impl.h"
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2 | #include <semaphore.h>
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3 | #include <unistd.h>
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4 | #include <dirent.h>
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5 | #include <string.h>
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6 | #include <ctype.h>
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7 | #include "futex.h"
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8 | #include "atomic.h"
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9 | #include "../dirent/__dirent.h"
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10 |
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11 | static struct chain {
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12 | struct chain *next;
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13 | int tid;
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14 | sem_t target_sem, caller_sem;
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15 | } *volatile head;
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16 |
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17 | static volatile int synccall_lock[2];
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18 | static volatile int target_tid;
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19 | static void (*callback)(void *), *context;
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20 | static volatile int dummy = 0;
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21 | weak_alias(dummy, __block_new_threads);
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22 |
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23 | static void handler(int sig)
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24 | {
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25 | struct chain ch;
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26 | int old_errno = errno;
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27 |
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28 | sem_init(&ch.target_sem, 0, 0);
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29 | sem_init(&ch.caller_sem, 0, 0);
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30 |
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31 | ch.tid = __syscall(SYS_gettid);
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32 |
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33 | do ch.next = head;
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34 | while (a_cas_p(&head, ch.next, &ch) != ch.next);
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35 |
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36 | if (a_cas(&target_tid, ch.tid, 0) == (ch.tid | 0x80000000))
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37 | __syscall(SYS_futex, &target_tid, FUTEX_UNLOCK_PI|FUTEX_PRIVATE);
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38 |
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39 | sem_wait(&ch.target_sem);
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40 | callback(context);
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41 | sem_post(&ch.caller_sem);
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42 | sem_wait(&ch.target_sem);
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43 |
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44 | errno = old_errno;
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45 | }
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46 |
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47 | void __synccall(void (*func)(void *), void *ctx)
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48 | {
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49 | sigset_t oldmask;
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50 | int cs, i, r, pid, self;;
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51 | DIR dir = {0};
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52 | struct dirent *de;
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53 | struct sigaction sa = { .sa_flags = SA_RESTART, .sa_handler = handler };
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54 | struct chain *cp, *next;
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55 | struct timespec ts;
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56 |
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57 | /* Blocking signals in two steps, first only app-level signals
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58 | * before taking the lock, then all signals after taking the lock,
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59 | * is necessary to achieve AS-safety. Blocking them all first would
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60 | * deadlock if multiple threads called __synccall. Waiting to block
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61 | * any until after the lock would allow re-entry in the same thread
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62 | * with the lock already held. */
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63 | __block_app_sigs(&oldmask);
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64 | LOCK(synccall_lock);
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65 | __block_all_sigs(0);
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66 | pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
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67 |
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68 | head = 0;
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69 |
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70 | if (!libc.threaded) goto single_threaded;
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71 |
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72 | callback = func;
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73 | context = ctx;
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74 |
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75 | /* This atomic store ensures that any signaled threads will see the
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76 | * above stores, and prevents more than a bounded number of threads,
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77 | * those already in pthread_create, from creating new threads until
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78 | * the value is cleared to zero again. */
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79 | a_store(&__block_new_threads, 1);
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80 |
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81 | /* Block even implementation-internal signals, so that nothing
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82 | * interrupts the SIGSYNCCALL handlers. The main possible source
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83 | * of trouble is asynchronous cancellation. */
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84 | memset(&sa.sa_mask, -1, sizeof sa.sa_mask);
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85 | __libc_sigaction(SIGSYNCCALL, &sa, 0);
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86 |
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87 | pid = __syscall(SYS_getpid);
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88 | self = __syscall(SYS_gettid);
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89 |
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90 | /* Since opendir is not AS-safe, the DIR needs to be setup manually
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91 | * in automatic storage. Thankfully this is easy. */
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92 | dir.fd = open("/proc/self/task", O_RDONLY|O_DIRECTORY|O_CLOEXEC);
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93 | if (dir.fd < 0) goto out;
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94 |
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95 | /* Initially send one signal per counted thread. But since we can't
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96 | * synchronize with thread creation/exit here, there could be too
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97 | * few signals. This initial signaling is just an optimization, not
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98 | * part of the logic. */
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99 | for (i=libc.threads_minus_1; i; i--)
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100 | __syscall(SYS_kill, pid, SIGSYNCCALL);
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101 |
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102 | /* Loop scanning the kernel-provided thread list until it shows no
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103 | * threads that have not already replied to the signal. */
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104 | for (;;) {
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105 | int miss_cnt = 0;
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106 | while ((de = readdir(&dir))) {
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107 | if (!isdigit(de->d_name[0])) continue;
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108 | int tid = atoi(de->d_name);
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109 | if (tid == self || !tid) continue;
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110 |
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111 | /* Set the target thread as the PI futex owner before
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112 | * checking if it's in the list of caught threads. If it
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113 | * adds itself to the list after we check for it, then
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114 | * it will see its own tid in the PI futex and perform
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115 | * the unlock operation. */
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116 | a_store(&target_tid, tid);
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117 |
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118 | /* Thread-already-caught is a success condition. */
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119 | for (cp = head; cp && cp->tid != tid; cp=cp->next);
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120 | if (cp) continue;
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121 |
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122 | r = -__syscall(SYS_tgkill, pid, tid, SIGSYNCCALL);
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123 |
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124 | /* Target thread exit is a success condition. */
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125 | if (r == ESRCH) continue;
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126 |
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127 | /* The FUTEX_LOCK_PI operation is used to loan priority
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128 | * to the target thread, which otherwise may be unable
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129 | * to run. Timeout is necessary because there is a race
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130 | * condition where the tid may be reused by a different
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131 | * process. */
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132 | clock_gettime(CLOCK_REALTIME, &ts);
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133 | ts.tv_nsec += 10000000;
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134 | if (ts.tv_nsec >= 1000000000) {
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135 | ts.tv_sec++;
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136 | ts.tv_nsec -= 1000000000;
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137 | }
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138 | r = -__syscall(SYS_futex, &target_tid,
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139 | FUTEX_LOCK_PI|FUTEX_PRIVATE, 0, &ts);
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140 |
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141 | /* Obtaining the lock means the thread responded. ESRCH
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142 | * means the target thread exited, which is okay too. */
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143 | if (!r || r == ESRCH) continue;
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144 |
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145 | miss_cnt++;
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146 | }
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147 | if (!miss_cnt) break;
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148 | rewinddir(&dir);
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149 | }
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150 | close(dir.fd);
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151 |
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152 | /* Serialize execution of callback in caught threads. */
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153 | for (cp=head; cp; cp=cp->next) {
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154 | sem_post(&cp->target_sem);
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155 | sem_wait(&cp->caller_sem);
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156 | }
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157 |
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158 | sa.sa_handler = SIG_IGN;
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159 | __libc_sigaction(SIGSYNCCALL, &sa, 0);
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160 |
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161 | single_threaded:
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162 | func(ctx);
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163 |
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164 | /* Only release the caught threads once all threads, including the
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165 | * caller, have returned from the callback function. */
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166 | for (cp=head; cp; cp=next) {
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167 | next = cp->next;
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168 | sem_post(&cp->target_sem);
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169 | }
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170 |
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171 | out:
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172 | a_store(&__block_new_threads, 0);
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173 | __wake(&__block_new_threads, -1, 1);
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174 |
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175 | pthread_setcancelstate(cs, 0);
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176 | UNLOCK(synccall_lock);
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177 | __restore_sigs(&oldmask);
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178 | }
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