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avoid accessing mutex memory after atomic unlock

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this change is needed to fix a race condition and ensure that it's
possible to unlock and destroy or unmap the mutex as soon as
pthread_mutex_lock succeeds. POSIX explicitly gives such an example in
the rationale and requires an implementation to allow such usage.
This commit is contained in:
Rich Felker
2011-08-02 20:31:15 -04:00
parent 344ea14885
commit c68de0be2f
4 changed files with 31 additions and 34 deletions
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14
src/thread/pthread_mutex_lock.c
View File

@ -2,17 +2,5 @@
int pthread_mutex_lock(pthread_mutex_t *m)
{
int r;
if (m->_m_type == PTHREAD_MUTEX_NORMAL && !a_swap(&m->_m_lock, EBUSY))
return 0;
while ((r=pthread_mutex_trylock(m)) == EBUSY) {
if (!(r=m->_m_lock) || (r&0x40000000)) continue;
if ((m->_m_type&3) == PTHREAD_MUTEX_ERRORCHECK
&& (r&0x1fffffff) == pthread_self()->tid)
return EDEADLK;
__wait(&m->_m_lock, &m->_m_waiters, r, 0);
}
return r;
return pthread_mutex_timedlock(m, 0);
}

22
src/thread/pthread_mutex_timedlock.c
View File

@ -2,15 +2,23 @@
int pthread_mutex_timedlock(pthread_mutex_t *m, const struct timespec *at)
{
int r, w=0;
int r, t;
if (m->_m_type == PTHREAD_MUTEX_NORMAL && !a_cas(&m->_m_lock, 0, EBUSY))
return 0;
while ((r=pthread_mutex_trylock(m)) == EBUSY) {
if (!(r=m->_m_lock) || (r&0x40000000)) continue;
if (!w) a_inc(&m->_m_waiters), w++;
if (__timedwait(&m->_m_lock, r, CLOCK_REALTIME, at, 0) == ETIMEDOUT) {
if (w) a_dec(&m->_m_waiters);
return ETIMEDOUT;
}
if ((m->_m_type&3) == PTHREAD_MUTEX_ERRORCHECK
&& (r&0x1fffffff) == pthread_self()->tid)
return EDEADLK;
a_inc(&m->_m_waiters);
t = r | 0x80000000;
a_cas(&m->_m_lock, r, t);
r = __timedwait(&m->_m_lock, t, CLOCK_REALTIME, at, 0);
a_dec(&m->_m_waiters);
if (r && r != EINTR) break;
}
if (w) a_dec(&m->_m_waiters);
return r;
}

14
src/thread/pthread_mutex_trylock.c
View File

@ -2,15 +2,14 @@
int pthread_mutex_trylock(pthread_mutex_t *m)
{
int tid;
int own;
int tid, old, own;
pthread_t self;
if (m->_m_type == PTHREAD_MUTEX_NORMAL)
return a_swap(&m->_m_lock, EBUSY);
return a_cas(&m->_m_lock, 0, EBUSY) & EBUSY;
self = pthread_self();
tid = self->tid | 0x80000000;
tid = self->tid;
if (m->_m_type >= 4) {
if (!self->robust_list.off)
@ -20,14 +19,15 @@ int pthread_mutex_trylock(pthread_mutex_t *m)
self->robust_list.pending = &m->_m_next;
}
if (m->_m_lock == tid && (m->_m_type&3) == PTHREAD_MUTEX_RECURSIVE) {
old = m->_m_lock;
own = old & 0x7fffffff;
if (own == tid && (m->_m_type&3) == PTHREAD_MUTEX_RECURSIVE) {
if ((unsigned)m->_m_count >= INT_MAX) return EAGAIN;
m->_m_count++;
return 0;
}
own = m->_m_lock;
if ((own && !(own & 0x40000000)) || a_cas(&m->_m_lock, own, tid)!=own)
if ((own && !(own & 0x40000000)) || a_cas(&m->_m_lock, old, tid)!=old)
return EBUSY;
m->_m_count = 1;

15
src/thread/pthread_mutex_unlock.c
View File

@ -3,6 +3,8 @@
int pthread_mutex_unlock(pthread_mutex_t *m)
{
pthread_t self;
int waiters = m->_m_waiters;
int cont;
if (m->_m_type != PTHREAD_MUTEX_NORMAL) {
if (!m->_m_lock)
@ -16,15 +18,14 @@ int pthread_mutex_unlock(pthread_mutex_t *m)
self->robust_list.pending = &m->_m_next;
*(void **)m->_m_prev = m->_m_next;
if (m->_m_next) ((void **)m->_m_next)[-1] = m->_m_prev;
a_store(&m->_m_lock, 0);
cont = a_swap(&m->_m_lock, 0);
self->robust_list.pending = 0;
} else {
a_store(&m->_m_lock, 0);
goto wake;
}
} else {
a_store(&m->_m_lock, 0);
}
if (m->_m_waiters) __wake(&m->_m_lock, 1, 0);
cont = a_swap(&m->_m_lock, 0);
wake:
if (waiters || cont<0)
__wake(&m->_m_lock, 1, 0);
return 0;
}