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,
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);
73