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-rw-r--r--src/thread/__lock.c55
1 files changed, 50 insertions, 5 deletions
diff --git a/src/thread/__lock.c b/src/thread/__lock.c
index 0874c04a..45557c88 100644
--- a/src/thread/__lock.c
+++ b/src/thread/__lock.c
@@ -1,15 +1,60 @@
#include "pthread_impl.h"
+/* 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 (libc.threads_minus_1)
- while (a_swap(l, 1)) __wait(l, l+1, 1, 1);
+ if (!libc.threads_minus_1) return;
+ /* fast path: INT_MIN for the lock, +1 for the congestion */
+ int current = a_cas(l, 0, INT_MIN + 1);
+ 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)
{
- if (l[0]) {
- a_store(l, 0);
- if (l[1]) __wake(l, 1, 1);
+ /* 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);
+ }
}
}