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
dummy_0(void)8 static void dummy_0(void)
9 {
10 }
11
dummy_1(pthread_t t)12 static void dummy_1(pthread_t t)
13 {
14 }
15
16 weak_alias(dummy_0, __tl_lock);
17 weak_alias(dummy_0, __tl_unlock);
18 weak_alias(dummy_1, __tl_sync);
19
20 static sem_t barrier_sem;
21
bcast_barrier(int s)22 static void bcast_barrier(int s)
23 {
24 sem_post(&barrier_sem);
25 }
26
__membarrier(int cmd,int flags)27 int __membarrier(int cmd, int flags)
28 {
29 int r = __syscall(SYS_membarrier, cmd, flags);
30 /* Emulate the private expedited command, which is needed by the
31 * dynamic linker for installation of dynamic TLS, for older
32 * kernels that lack the syscall. Unlike the syscall, this only
33 * synchronizes with threads of the process, not other processes
34 * sharing the VM, but such sharing is not a supported usage
35 * anyway. */
36 if (r && cmd == MEMBARRIER_CMD_PRIVATE_EXPEDITED && !flags) {
37 pthread_t self=__pthread_self(), td;
38 sigset_t set;
39 __block_app_sigs(&set);
40 __tl_lock();
41 sem_init(&barrier_sem, 0, 0);
42 struct sigaction sa = {
43 .sa_flags = SA_RESTART,
44 .sa_handler = bcast_barrier
45 };
46 memset(&sa.sa_mask, -1, sizeof sa.sa_mask);
47 if (!__libc_sigaction(SIGSYNCCALL, &sa, 0)) {
48 for (td=self->next; td!=self; td=td->next)
49 __syscall(SYS_tkill, td->tid, SIGSYNCCALL);
50 for (td=self->next; td!=self; td=td->next)
51 sem_wait(&barrier_sem);
52 r = 0;
53 sa.sa_handler = SIG_IGN;
54 __libc_sigaction(SIGSYNCCALL, &sa, 0);
55 }
56 sem_destroy(&barrier_sem);
57 __tl_unlock();
58 __restore_sigs(&set);
59 }
60 return __syscall_ret(r);
61 }
62
__membarrier_init(void)63 void __membarrier_init(void)
64 {
65 /* If membarrier is linked, attempt to pre-register to be able to use
66 * the private expedited command before the process becomes multi-
67 * threaded, since registering later has bad, potentially unbounded
68 * latency. This syscall should be essentially free, and it's arguably
69 * a mistake in the API design that registration was even required.
70 * For other commands, registration may impose some cost, so it's left
71 * to the application to do so if desired. Unfortunately this means
72 * library code initialized after the process becomes multi-threaded
73 * cannot use these features without accepting registration latency. */
74 __syscall(SYS_membarrier, MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED, 0);
75 }
76
77 weak_alias(__membarrier, membarrier);
78