redesign and simplify vmlock system

this global lock allows certain unlock-type primitives to exclude
mmap/munmap operations which could change the identity of virtual
addresses while references to them still exist.

the original design mistakenly assumed mmap/munmap would conversely
need to exclude the same operations which exclude mmap/munmap, so the
vmlock was implemented as a sort of 'symmetric recursive rwlock'. this
turned out to be unnecessary.

commit 25d12fc0fc51f1fae0f85b4649a6463eb805aa8f already shortened the
interval during which mmap/munmap held their side of the lock, but
left the inappropriate lock design and some inefficiency.

the new design uses a separate function, __vm_wait, which does not
hold any lock itself and only waits for lock users which were already
present when it was called to release the lock. this is sufficient
because of the way operations that need to be excluded are sequenced:
the "unlock-type" operations using the vmlock need only block
mmap/munmap operations that are precipitated by (and thus sequenced
after) the atomic-unlock they perform while holding the vmlock.

this allows for a spectacular lack of synchronization in the __vm_wait
function itself.

src/internal/pthread_impl.h

@@ -108,6 +108,10 @@ int __libc_sigprocmask(int, const sigset_t *, sigset_t *);
 void __lock(volatile int *);
 void __unmapself(void *, size_t);
 
+void __vm_wait();
+void __vm_lock();
+void __vm_unlock();
+
 int __timedwait(volatile int *, int, clockid_t, const struct timespec *, int);
 int __timedwait_cp(volatile int *, int, clockid_t, const struct timespec *, int);
 void __wait(volatile int *, volatile int *, int, int);

src/mman/mmap.c

@@ -6,10 +6,8 @@
 #include "syscall.h"
 #include "libc.h"
 
-static void dummy1(int x) { }
-static void dummy0(void) { }
-weak_alias(dummy1, __vm_lock);
-weak_alias(dummy0, __vm_unlock);
+static void dummy(void) { }
+weak_alias(dummy, __vm_wait);
 
 #define UNIT SYSCALL_MMAP2_UNIT
 #define OFF_MASK ((-0x2000ULL << (8*sizeof(long)-1)) | (UNIT-1))
@@ -25,8 +23,7 @@ void *__mmap(void *start, size_t len, int prot, int flags, int fd, off_t off)
 		return MAP_FAILED;
 	}
 	if (flags & MAP_FIXED) {
-		__vm_lock(-1);
-		__vm_unlock();
+		__vm_wait();
 	}
 #ifdef SYS_mmap2
 	return (void *)syscall(SYS_mmap2, start, len, prot, flags, fd, off/UNIT);

src/mman/munmap.c

@@ -2,18 +2,13 @@
 #include "syscall.h"
 #include "libc.h"
 
-static void dummy1(int x) { }
-static void dummy0(void) { }
-weak_alias(dummy1, __vm_lock);
-weak_alias(dummy0, __vm_unlock);
+static void dummy(void) { }
+weak_alias(dummy, __vm_wait);
 
 int __munmap(void *start, size_t len)
 {
-	int ret;
-	__vm_lock(-1);
-	__vm_unlock();
-	ret = syscall(SYS_munmap, start, len);
-	return ret;
+	__vm_wait();
+	return syscall(SYS_munmap, start, len);
 }
 
 weak_alias(__munmap, munmap);

src/thread/pthread_barrier_destroy.c

@@ -1,7 +1,5 @@
 #include "pthread_impl.h"
 
-void __vm_lock(int), __vm_unlock(void);
-
 int pthread_barrier_destroy(pthread_barrier_t *b)
 {
 	if (b->_b_limit < 0) {
@@ -11,8 +9,7 @@ int pthread_barrier_destroy(pthread_barrier_t *b)
 			while ((v = b->_b_lock) & INT_MAX)
 				__wait(&b->_b_lock, 0, v, 0);
 		}
-		__vm_lock(-1);
-		__vm_unlock();
+		__vm_wait();
 	}
 	return 0;
 }

src/thread/pthread_barrier_wait.c

@@ -1,8 +1,5 @@
 #include "pthread_impl.h"
 
-void __vm_lock_impl(int);
-void __vm_unlock_impl(void);
-
 static int pshared_barrier_wait(pthread_barrier_t *b)
 {
 	int limit = (b->_b_limit & INT_MAX) + 1;
@@ -26,7 +23,7 @@ static int pshared_barrier_wait(pthread_barrier_t *b)
 			__wait(&b->_b_count, &b->_b_waiters2, v, 0);
 	}
 
-	__vm_lock_impl(+1);
+	__vm_lock();
 
 	/* Ensure all threads have a vm lock before proceeding */
 	if (a_fetch_add(&b->_b_count, -1)==1-limit) {
@@ -47,7 +44,7 @@ static int pshared_barrier_wait(pthread_barrier_t *b)
 	if (v==INT_MIN+1 || (v==1 && w))
 		__wake(&b->_b_lock, 1, 0);
 
-	__vm_unlock_impl();
+	__vm_unlock();
 
 	return ret;
 }

src/thread/pthread_create.c

@@ -9,8 +9,6 @@
 void *__mmap(void *, size_t, int, int, int, off_t);
 int __munmap(void *, size_t);
 int __mprotect(void *, size_t, int);
-void __vm_lock_impl(int);
-void __vm_unlock_impl(void);
 
 static void dummy_0()
 {
@@ -77,7 +75,7 @@ _Noreturn void __pthread_exit(void *result)
 	/* Process robust list in userspace to handle non-pshared mutexes
 	 * and the detached thread case where the robust list head will
 	 * be invalid when the kernel would process it. */
-	__vm_lock_impl(+1);
+	__vm_lock();
 	volatile void *volatile *rp;
 	while ((rp=self->robust_list.head) && rp != &self->robust_list.head) {
 		pthread_mutex_t *m = (void *)((char *)rp
@@ -91,7 +89,7 @@ _Noreturn void __pthread_exit(void *result)
 		if (cont < 0 || waiters)
 			__wake(&m->_m_lock, 1, priv);
 	}
-	__vm_unlock_impl();
+	__vm_unlock();
 
 	__do_orphaned_stdio_locks();
 

src/thread/pthread_mutex_unlock.c

@@ -1,8 +1,5 @@
 #include "pthread_impl.h"
 
-void __vm_lock_impl(int);
-void __vm_unlock_impl(void);
-
 int __pthread_mutex_unlock(pthread_mutex_t *m)
 {
 	pthread_t self;
@@ -19,7 +16,7 @@ int __pthread_mutex_unlock(pthread_mutex_t *m)
 			return m->_m_count--, 0;
 		if (!priv) {
 			self->robust_list.pending = &m->_m_next;
-			__vm_lock_impl(+1);
+			__vm_lock();
 		}
 		volatile void *prev = m->_m_prev;
 		volatile void *next = m->_m_next;
@@ -30,7 +27,7 @@ int __pthread_mutex_unlock(pthread_mutex_t *m)
 	cont = a_swap(&m->_m_lock, (type & 8) ? 0x40000000 : 0);
 	if (type != PTHREAD_MUTEX_NORMAL && !priv) {
 		self->robust_list.pending = 0;
-		__vm_unlock_impl();
+		__vm_unlock();
 	}
 	if (waiters || cont<0)
 		__wake(&m->_m_lock, 1, priv);

src/thread/vmlock.c

@@ -2,21 +2,20 @@
 
 static volatile int vmlock[2];
 
-void __vm_lock(int inc)
+void __vm_wait()
 {
-	for (;;) {
-		int v = vmlock[0];
-		if (inc*v < 0) __wait(vmlock, vmlock+1, v, 1);
-		else if (a_cas(vmlock, v, v+inc)==v) break;
-	}
+	int tmp;
+	while ((tmp=vmlock[0]))
+		__wait(vmlock, vmlock+1, tmp, 1);
 }
 
-void __vm_unlock(void)
+void __vm_lock(pthread_t self)
 {
-	int inc = vmlock[0]>0 ? -1 : 1;
-	if (a_fetch_add(vmlock, inc)==-inc && vmlock[1])
+	a_inc(vmlock);
+}
+
+void __vm_unlock(pthread_t self)
+{
+	if (a_fetch_add(vmlock, -1)==1 && vmlock[1])
 		__wake(vmlock, -1, 1);
 }
-
-weak_alias(__vm_lock, __vm_lock_impl);
-weak_alias(__vm_unlock, __vm_unlock_impl);