#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) 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)
{
/* 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);
}
}
}