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https://github.com/fluencelabs/musl
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384d103d94
Linux's documentation (robust-futex-ABI.txt) claims that, when a process dies with a futex on the robust list, bit 30 (0x40000000) is set to indicate the status. however, what actually happens is that bits 0-30 are replaced with the value 0x40000000, i.e. bits 0-29 (containing the old owner tid) are cleared at the same time bit 30 is set. our userspace-side code for robust mutexes was written based on that documentation, assuming that kernel would never produce a futex value of 0x40000000, since the low (owner) bits would always be non-zero. commit d338b506e39b1e2c68366b12be90704c635602ce introduced this assumption explicitly while fixing another bug in how non-recoverable status for robust mutexes was tracked. presumably the tests conducted at that time only checked non-process-shared robust mutexes, which are handled in pthread_exit (which implemented the documented kernel protocol, not the actual one) rather than by the kernel. change pthread_exit robust list processing to match the kernel behavior, clearing bits 0-29 while setting bit 30, and use the value 0x7fffffff instead of 0x40000000 to encode non-recoverable status. the choice of value here is arbitrary; any value with at least one of bits 0-29 set should work just as well,
59 lines
1.5 KiB
C
59 lines
1.5 KiB
C
#include "pthread_impl.h"
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int __pthread_mutex_trylock_owner(pthread_mutex_t *m)
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{
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int old, own;
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int type = m->_m_type & 15;
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pthread_t self = __pthread_self();
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int tid = self->tid;
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old = m->_m_lock;
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own = old & 0x7fffffff;
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if (own == tid && (type&3) == PTHREAD_MUTEX_RECURSIVE) {
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if ((unsigned)m->_m_count >= INT_MAX) return EAGAIN;
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m->_m_count++;
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return 0;
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}
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if (own == 0x7fffffff) return ENOTRECOVERABLE;
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if (m->_m_type & 128) {
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if (!self->robust_list.off) {
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self->robust_list.off = (char*)&m->_m_lock-(char *)&m->_m_next;
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__syscall(SYS_set_robust_list, &self->robust_list, 3*sizeof(long));
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}
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if (m->_m_waiters) tid |= 0x80000000;
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self->robust_list.pending = &m->_m_next;
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}
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if ((own && (!(own & 0x40000000) || !(type & 4)))
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|| a_cas(&m->_m_lock, old, tid) != old) {
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self->robust_list.pending = 0;
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return EBUSY;
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}
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volatile void *next = self->robust_list.head;
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m->_m_next = next;
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m->_m_prev = &self->robust_list.head;
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if (next != &self->robust_list.head) *(volatile void *volatile *)
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((char *)next - sizeof(void *)) = &m->_m_next;
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self->robust_list.head = &m->_m_next;
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self->robust_list.pending = 0;
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if (own) {
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m->_m_count = 0;
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m->_m_type |= 8;
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return EOWNERDEAD;
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}
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return 0;
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}
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int __pthread_mutex_trylock(pthread_mutex_t *m)
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{
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if ((m->_m_type&15) == PTHREAD_MUTEX_NORMAL)
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return a_cas(&m->_m_lock, 0, EBUSY) & EBUSY;
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return __pthread_mutex_trylock_owner(m);
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}
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weak_alias(__pthread_mutex_trylock, pthread_mutex_trylock);
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