73 lines
2.2 KiB
C
73 lines
2.2 KiB
C
#include <sys/membarrier.h>
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#include <semaphore.h>
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#include <signal.h>
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#include <string.h>
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#include "pthread_impl.h"
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#include "syscall.h"
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static void dummy_0(void)
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{
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}
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weak_alias(dummy_0, __tl_lock);
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weak_alias(dummy_0, __tl_unlock);
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static sem_t barrier_sem;
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static void bcast_barrier(int s)
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{
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sem_post(&barrier_sem);
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}
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int __membarrier(int cmd, int flags)
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{
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int r = __syscall(SYS_membarrier, cmd, flags);
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/* Emulate the private expedited command, which is needed by the
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* dynamic linker for installation of dynamic TLS, for older
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* kernels that lack the syscall. Unlike the syscall, this only
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* synchronizes with threads of the process, not other processes
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* sharing the VM, but such sharing is not a supported usage
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* anyway. */
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if (r && cmd == MEMBARRIER_CMD_PRIVATE_EXPEDITED && !flags) {
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pthread_t self=__pthread_self(), td;
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sigset_t set;
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__block_app_sigs(&set);
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__tl_lock();
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sem_init(&barrier_sem, 0, 0);
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struct sigaction sa = {
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.sa_flags = SA_RESTART,
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.sa_handler = bcast_barrier
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};
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memset(&sa.sa_mask, -1, sizeof sa.sa_mask);
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if (!__libc_sigaction(SIGSYNCCALL, &sa, 0)) {
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for (td=self->next; td!=self; td=td->next)
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__syscall(SYS_tkill, td->tid, SIGSYNCCALL);
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for (td=self->next; td!=self; td=td->next)
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sem_wait(&barrier_sem);
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r = 0;
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sa.sa_handler = SIG_IGN;
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__libc_sigaction(SIGSYNCCALL, &sa, 0);
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}
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sem_destroy(&barrier_sem);
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__tl_unlock();
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__restore_sigs(&set);
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}
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return __syscall_ret(r);
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}
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void __membarrier_init(void)
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{
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/* If membarrier is linked, attempt to pre-register to be able to use
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* the private expedited command before the process becomes multi-
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* threaded, since registering later has bad, potentially unbounded
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* latency. This syscall should be essentially free, and it's arguably
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* a mistake in the API design that registration was even required.
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* For other commands, registration may impose some cost, so it's left
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* to the application to do so if desired. Unfortunately this means
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* library code initialized after the process becomes multi-threaded
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* cannot use these features without accepting registration latency. */
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__syscall(SYS_membarrier, MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED, 0);
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}
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weak_alias(__membarrier, membarrier);
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