src/linux/membarrier.c

   1 #include <sys/membarrier.h>
   2 #include <semaphore.h>
   3 #include <signal.h>
   4 #include <string.h>
   5 #include "pthread_impl.h"
   6 #include "syscall.h"
   7
   8 static void dummy_0(void)
   9 {
  10 }
  11
  12 weak_alias(dummy_0, __tl_lock);
  13 weak_alias(dummy_0, __tl_unlock);
  14
  15 static sem_t barrier_sem;
  16
  17 static void bcast_barrier(int s)
  18 {
  19         sem_post(&barrier_sem);
  20 }
  21
  22 int __membarrier(int cmd, int flags)
  23 {
  24         int r = __syscall(SYS_membarrier, cmd, flags);
  25         /* Emulate the private expedited command, which is needed by the
  26          * dynamic linker for installation of dynamic TLS, for older
  27          * kernels that lack the syscall. Unlike the syscall, this only
  28          * synchronizes with threads of the process, not other processes
  29          * sharing the VM, but such sharing is not a supported usage
  30          * anyway. */
  31         if (r && cmd == MEMBARRIER_CMD_PRIVATE_EXPEDITED && !flags) {
  32                 pthread_t self=__pthread_self(), td;
  33                 sigset_t set;
  34                 __block_app_sigs(&set);
  35                 __tl_lock();
  36                 sem_init(&barrier_sem, 0, 0);
  37                 struct sigaction sa = {
  38                         .sa_flags = SA_RESTART | SA_ONSTACK,
  39                         .sa_handler = bcast_barrier
  40                 };
  41                 memset(&sa.sa_mask, -1, sizeof sa.sa_mask);
  42                 if (!__libc_sigaction(SIGSYNCCALL, &sa, 0)) {
  43                         for (td=self->next; td!=self; td=td->next)
  44                                 __syscall(SYS_tkill, td->tid, SIGSYNCCALL);
  45                         for (td=self->next; td!=self; td=td->next)
  46                                 sem_wait(&barrier_sem);
  47                         r = 0;
  48                         sa.sa_handler = SIG_IGN;
  49                         __libc_sigaction(SIGSYNCCALL, &sa, 0);
  50                 }
  51                 sem_destroy(&barrier_sem);
  52                 __tl_unlock();
  53                 __restore_sigs(&set);
  54         }
  55         return __syscall_ret(r);
  56 }
  57
  58 void __membarrier_init(void)
  59 {
  60         /* If membarrier is linked, attempt to pre-register to be able to use
  61          * the private expedited command before the process becomes multi-
  62          * threaded, since registering later has bad, potentially unbounded
  63          * latency. This syscall should be essentially free, and it's arguably
  64          * a mistake in the API design that registration was even required.
  65          * For other commands, registration may impose some cost, so it's left
  66          * to the application to do so if desired. Unfortunately this means
  67          * library code initialized after the process becomes multi-threaded
  68          * cannot use these features without accepting registration latency. */
  69         __syscall(SYS_membarrier, MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED, 0);
  70 }
  71
  72 weak_alias(__membarrier, membarrier);