diff --git a/src/thread/pthread_mutex_timedlock.c b/src/thread/pthread_mutex_timedlock.c
index 0f171c00..6b893627 100644
--- a/src/thread/pthread_mutex_timedlock.c
+++ b/src/thread/pthread_mutex_timedlock.c
@@ -1,5 +1,40 @@
 #include "pthread_impl.h"
 
+static int pthread_mutex_timedlock_pi(pthread_mutex_t *restrict m, const struct timespec *restrict at)
+{
+	int type = m->_m_type;
+	int priv = (type & 128) ^ 128;
+	pthread_t self = __pthread_self();
+	int e;
+
+	if (!priv) self->robust_list.pending = &m->_m_next;
+
+	do e = -__syscall(SYS_futex, &m->_m_lock, FUTEX_LOCK_PI|priv, 0, at);
+	while (e==EINTR);
+	if (e) self->robust_list.pending = 0;
+
+	switch (e) {
+	case 0:
+		/* Catch spurious success for non-robust mutexes. */
+		if (!(type&4) && ((m->_m_lock & 0x40000000) || m->_m_waiters)) {
+			a_store(&m->_m_waiters, -1);
+			__syscall(SYS_futex, &m->_m_lock, FUTEX_UNLOCK_PI|priv);
+			self->robust_list.pending = 0;
+			break;
+		}
+		/* Signal to trylock that we already have the lock. */
+		m->_m_count = -1;
+		return __pthread_mutex_trylock(m);
+	case ETIMEDOUT:
+		return e;
+	case EDEADLK:
+		if ((type&3) == PTHREAD_MUTEX_ERRORCHECK) return e;
+	}
+	do e = __timedwait(&(int){0}, 0, CLOCK_REALTIME, at, 1);
+	while (e != ETIMEDOUT);
+	return e;
+}
+
 int __pthread_mutex_timedlock(pthread_mutex_t *restrict m, const struct timespec *restrict at)
 {
 	if ((m->_m_type&15) == PTHREAD_MUTEX_NORMAL
@@ -11,6 +46,8 @@ int __pthread_mutex_timedlock(pthread_mutex_t *restrict m, const struct timespec
 
 	r = __pthread_mutex_trylock(m);
 	if (r != EBUSY) return r;
+
+	if (type&8) return pthread_mutex_timedlock_pi(m, at);
 	
 	int spins = 100;
 	while (spins-- && m->_m_lock && !m->_m_waiters) a_spin();
diff --git a/src/thread/pthread_mutex_trylock.c b/src/thread/pthread_mutex_trylock.c
index 29622ff9..37e5c473 100644
--- a/src/thread/pthread_mutex_trylock.c
+++ b/src/thread/pthread_mutex_trylock.c
@@ -9,10 +9,17 @@ int __pthread_mutex_trylock_owner(pthread_mutex_t *m)
 
 	old = m->_m_lock;
 	own = old & 0x3fffffff;
-	if (own == tid && (type&3) == PTHREAD_MUTEX_RECURSIVE) {
-		if ((unsigned)m->_m_count >= INT_MAX) return EAGAIN;
-		m->_m_count++;
-		return 0;
+	if (own == tid) {
+		if ((type&8) && m->_m_count<0) {
+			old &= 0x40000000;
+			m->_m_count = 0;
+			goto success;
+		}
+		if ((type&3) == PTHREAD_MUTEX_RECURSIVE) {
+			if ((unsigned)m->_m_count >= INT_MAX) return EAGAIN;
+			m->_m_count++;
+			return 0;
+		}
 	}
 	if (own == 0x3fffffff) return ENOTRECOVERABLE;
 	if (own || (old && !(type & 4))) return EBUSY;
@@ -29,9 +36,18 @@ int __pthread_mutex_trylock_owner(pthread_mutex_t *m)
 
 	if (a_cas(&m->_m_lock, old, tid) != old) {
 		self->robust_list.pending = 0;
+		if ((type&12)==12 & m->_m_waiters) return ENOTRECOVERABLE;
 		return EBUSY;
 	}
 
+success:
+	if ((type&8) && m->_m_waiters) {
+		int priv = (type & 128) ^ 128;
+		__syscall(SYS_futex, &m->_m_lock, FUTEX_UNLOCK_PI|priv);
+		self->robust_list.pending = 0;
+		return (type&4) ? ENOTRECOVERABLE : EBUSY;
+	}
+
 	volatile void *next = self->robust_list.head;
 	m->_m_next = next;
 	m->_m_prev = &self->robust_list.head;
diff --git a/src/thread/pthread_mutex_unlock.c b/src/thread/pthread_mutex_unlock.c
index ea9f54dd..b66423e6 100644
--- a/src/thread/pthread_mutex_unlock.c
+++ b/src/thread/pthread_mutex_unlock.c
@@ -8,10 +8,11 @@ int __pthread_mutex_unlock(pthread_mutex_t *m)
 	int type = m->_m_type & 15;
 	int priv = (m->_m_type & 128) ^ 128;
 	int new = 0;
+	int old;
 
 	if (type != PTHREAD_MUTEX_NORMAL) {
 		self = __pthread_self();
-		int old = m->_m_lock;
+		old = m->_m_lock;
 		int own = old & 0x3fffffff;
 		if (own != self->tid)
 			return EPERM;
@@ -29,7 +30,16 @@ int __pthread_mutex_unlock(pthread_mutex_t *m)
 		if (next != &self->robust_list.head) *(volatile void *volatile *)
 			((char *)next - sizeof(void *)) = prev;
 	}
-	cont = a_swap(&m->_m_lock, new);
+	if (type&8) {
+		if (old<0 || a_cas(&m->_m_lock, old, new)!=old) {
+			if (new) a_store(&m->_m_waiters, -1);
+			__syscall(SYS_futex, &m->_m_lock, FUTEX_UNLOCK_PI|priv);
+		}
+		cont = 0;
+		waiters = 0;
+	} else {
+		cont = a_swap(&m->_m_lock, new);
+	}
 	if (type != PTHREAD_MUTEX_NORMAL && !priv) {
 		self->robust_list.pending = 0;
 		__vm_unlock();
diff --git a/src/thread/pthread_mutexattr_setprotocol.c b/src/thread/pthread_mutexattr_setprotocol.c
index c92a31c8..511cc32d 100644
--- a/src/thread/pthread_mutexattr_setprotocol.c
+++ b/src/thread/pthread_mutexattr_setprotocol.c
@@ -1,7 +1,29 @@
 #include "pthread_impl.h"
+#include "syscall.h"
+
+static pthread_once_t check_pi_once;
+static int check_pi_result;
+
+static void check_pi()
+{
+	volatile int lk = 0;
+	check_pi_result = -__syscall(SYS_futex, &lk, FUTEX_LOCK_PI, 0, 0);
+}
 
 int pthread_mutexattr_setprotocol(pthread_mutexattr_t *a, int protocol)
 {
-	if (protocol) return ENOTSUP;
-	return 0;
+	switch (protocol) {
+	case PTHREAD_PRIO_NONE:
+		a->__attr &= ~8;
+		return 0;
+	case PTHREAD_PRIO_INHERIT:
+		pthread_once(&check_pi_once, check_pi);
+		if (check_pi_result) return check_pi_result;
+		a->__attr |= 8;
+		return 0;
+	case PTHREAD_PRIO_PROTECT:
+		return ENOTSUP;
+	default:
+		return EINVAL;
+	}
 }