src/thread/__lock.c

   1 #include "pthread_impl.h"
   2
   3 /* This lock primitive combines a flag (in the sign bit) and a
   4  * congestion count (= threads inside the critical section, CS) in a
   5  * single int that is accessed through atomic operations. The states
   6  * of the int for value x are:
   7  *
   8  * x == 0: unlocked and no thread inside the critical section
   9  *
  10  * x < 0: locked with a congestion of x-INT_MIN, including the thread
  11  * that holds the lock
  12  *
  13  * x > 0: unlocked with a congestion of x
  14  *
  15  * or in an equivalent formulation x is the congestion count or'ed
  16  * with INT_MIN as a lock flag.
  17  */
  18
  19 void __lock(volatile int *l)
  20 {
  21         if (!libc.threads_minus_1) return;
  22         /* fast path: INT_MIN for the lock, +1 for the congestion */
  23         int current = a_cas(l, 0, INT_MIN + 1);
  24         if (!current) return;
  25         /* A first spin loop, for medium congestion. */
  26         for (unsigned i = 0; i < 10; ++i) {
  27                 if (current < 0) current -= INT_MIN + 1;
  28                 // assertion: current >= 0
  29                 int val = a_cas(l, current, INT_MIN + (current + 1));
  30                 if (val == current) return;
  31                 current = val;
  32         }
  33         // Spinning failed, so mark ourselves as being inside the CS.
  34         current = a_fetch_add(l, 1) + 1;
  35         /* The main lock acquisition loop for heavy congestion. The only
  36          * change to the value performed inside that loop is a successful
  37          * lock via the CAS that acquires the lock. */
  38         for (;;) {
  39                 /* We can only go into wait, if we know that somebody holds the
  40                  * lock and will eventually wake us up, again. */
  41                 if (current < 0) {
  42                         __futexwait(l, current, 1);
  43                         current -= INT_MIN + 1;
  44                 }
  45                 /* assertion: current > 0, the count includes us already. */
  46                 int val = a_cas(l, current, INT_MIN + current);
  47                 if (val == current) return;
  48                 current = val;
  49         }
  50 }
  51
  52 void __unlock(volatile int *l)
  53 {
  54         /* Check l[0] to see if we are multi-threaded. */
  55         if (l[0] < 0) {
  56                 if (a_fetch_add(l, -(INT_MIN + 1)) != (INT_MIN + 1)) {
  57                         __wake(l, 1, 1);
  58                 }
  59         }
  60 }