X-Git-Url: http://nsz.repo.hu/git/?a=blobdiff_plain;f=src%2Fthread%2F__lock.c;h=60eece49a28888dc3dd1a4498c0c6d9dd85effd4;hb=233bb6972d84e9cb908ff038f78d97e487082225;hp=5ba5dc5ee618a2fce5561f5a0f0a3e1709838719;hpb=e7655ed37bc9c2d79d921af4f287ee5cf2788661;p=musl

diff --git a/src/thread/__lock.c b/src/thread/__lock.c
index 5ba5dc5e..60eece49 100644
--- a/src/thread/__lock.c
+++ b/src/thread/__lock.c
@@ -1,32 +1,62 @@
 #include "pthread_impl.h"
 
-void __lock_2(volatile int *l)
-{
-	if (!__syscall(SYS_futex, l, FUTEX_LOCK_PI, 0, 0))
-		return;
-	int old, tid = __pthread_self()->tid|INT_MIN;
-	while ((old = a_cas(l, 0, tid))) {
-		a_cas(l, old, old|INT_MIN);
-		__syscall(SYS_futex, l, FUTEX_WAIT, old|INT_MIN, 0);
-	}
-}
+/* This lock primitive combines a flag (in the sign bit) and a
+ * congestion count (= threads inside the critical section, CS) in a
+ * single int that is accessed through atomic operations. The states
+ * of the int for value x are:
+ *
+ * x == 0: unlocked and no thread inside the critical section
+ *
+ * x < 0: locked with a congestion of x-INT_MIN, including the thread
+ * that holds the lock
+ *
+ * x > 0: unlocked with a congestion of x
+ *
+ * or in an equivalent formulation x is the congestion count or'ed
+ * with INT_MIN as a lock flag.
+ */
 
 void __lock(volatile int *l)
 {
-	if (a_cas(l, 0, __pthread_self()->tid)) __lock_2(l);
-}
-
-void __unlock_2(volatile int *l)
-{
-	if (__syscall(SYS_futex, l, FUTEX_UNLOCK_PI)) {
-		*l = 0;
-		__syscall(SYS_futex, l, FUTEX_WAKE, 1);
+	int need_locks = libc.need_locks;
+	if (!need_locks) return;
+	/* fast path: INT_MIN for the lock, +1 for the congestion */
+	int current = a_cas(l, 0, INT_MIN + 1);
+	if (need_locks < 0) libc.need_locks = 0;
+	if (!current) return;
+	/* A first spin loop, for medium congestion. */
+	for (unsigned i = 0; i < 10; ++i) {
+		if (current < 0) current -= INT_MIN + 1;
+		// assertion: current >= 0
+		int val = a_cas(l, current, INT_MIN + (current + 1));
+		if (val == current) return;
+		current = val;
+	}
+	// Spinning failed, so mark ourselves as being inside the CS.
+	current = a_fetch_add(l, 1) + 1;
+	/* The main lock acquisition loop for heavy congestion. The only
+	 * change to the value performed inside that loop is a successful
+	 * lock via the CAS that acquires the lock. */
+	for (;;) {
+		/* We can only go into wait, if we know that somebody holds the
+		 * lock and will eventually wake us up, again. */
+		if (current < 0) {
+			__futexwait(l, current, 1);
+			current -= INT_MIN + 1;
+		}
+		/* assertion: current > 0, the count includes us already. */
+		int val = a_cas(l, current, INT_MIN + current);
+		if (val == current) return;
+		current = val;
 	}
 }
 
 void __unlock(volatile int *l)
 {
-	int old = *l;
-	if (!(old & INT_MIN) && a_cas(l, old, 0)==old) return;
-	__unlock_2(l);
+	/* Check l[0] to see if we are multi-threaded. */
+	if (l[0] < 0) {
+		if (a_fetch_add(l, -(INT_MIN + 1)) != (INT_MIN + 1)) {
+			__wake(l, 1, 1);
+		}
+	}
 }