MFC r301157:

    Microoptimize locking primitives by avoiding unnecessary atomic ops.

    Inline version of primitives do an atomic op and if it fails they fallback to
    actual primitives, which immediately retry the atomic op.

    The obvious optimisation is to check if the lock is free and only then proceed
    to do an atomic op.

1 files changed, 9 insertions, 4 deletions

diff --git a/sys/kern/kern_mutex.c b/sys/kern/kern_mutex.c
index 8d19f2e..6107e79 100644
--- a/sys/kern/kern_mutex.c
+++ b/sys/kern/kern_mutex.c
@@ -451,7 +451,9 @@ __mtx_lock_sleep(volatile uintptr_t *c, uintptr_t tid, int opts,
 	all_time -= lockstat_nsecs(&m->lock_object);
 #endif
 
-	while (!_mtx_obtain_lock(m, tid)) {
+	for (;;) {
+		if (m->mtx_lock == MTX_UNOWNED && _mtx_obtain_lock(m, tid))
+			break;
 #ifdef KDTRACE_HOOKS
 		spin_cnt++;
 #endif
@@ -634,8 +636,9 @@ _mtx_lock_spin_cookie(volatile uintptr_t *c, uintptr_t tid, int opts,
 #ifdef KDTRACE_HOOKS
 	spin_time -= lockstat_nsecs(&m->lock_object);
 #endif
-	while (!_mtx_obtain_lock(m, tid)) {
-
+	for (;;) {
+		if (m->mtx_lock == MTX_UNOWNED && _mtx_obtain_lock(m, tid))
+			break;
 		/* Give interrupts a chance while we spin. */
 		spinlock_exit();
 		while (m->mtx_lock != MTX_UNOWNED) {
@@ -714,7 +717,9 @@ retry:
 			    m->lock_object.lo_name, file, line));
 		WITNESS_CHECKORDER(&m->lock_object,
 		    opts | LOP_NEWORDER | LOP_EXCLUSIVE, file, line, NULL);
-		while (!_mtx_obtain_lock(m, tid)) {
+		for (;;) {
+			if (m->mtx_lock == MTX_UNOWNED && _mtx_obtain_lock(m, tid))
+				break;
 			if (m->mtx_lock == tid) {
 				m->mtx_recurse++;
 				break;