libc-top-half/musl/src/thread/__wait.c

   1 #include "pthread_impl.h"
   2 #ifndef __wasilibc_unmodified_upstream
   3 #include "assert.h"
   4 #endif
   5
   6 #ifndef __wasilibc_unmodified_upstream
   7 // Use WebAssembly's `wait` instruction to implement a futex. Note that `op` is
   8 // unused but retained as a parameter to match the original signature of the
   9 // syscall and that, for `max_wait_ns`, -1 (or any negative number) means wait
  10 // indefinitely.
  11 //
  12 // Adapted from Emscripten: see
  13 // https://github.com/emscripten-core/emscripten/blob/058a9fff/system/lib/pthread/emscripten_futex_wait.c#L111-L150.
  14 int __wasilibc_futex_wait(volatile void *addr, int op, int val, int64_t max_wait_ns)
  15 {
  16     if ((((intptr_t)addr) & 3) != 0) {
  17         return -EINVAL;
  18     }
  19
  20     int ret = __builtin_wasm_memory_atomic_wait32((int *)addr, val, max_wait_ns);
  21
  22     // memory.atomic.wait32 returns:
  23     //   0 => "ok", woken by another agent.
  24     //   1 => "not-equal", loaded value != expected value
  25     //   2 => "timed-out", the timeout expired
  26     if (ret == 1) {
  27         return -EWOULDBLOCK;
  28     }
  29     if (ret == 2) {
  30         return -ETIMEDOUT;
  31     }
  32     assert(ret == 0);
  33     return 0;
  34 }
  35 #endif
  36
  37 void __wait(volatile int *addr, volatile int *waiters, int val, int priv)
  38 {
  39         int spins=100;
  40         if (priv) priv = FUTEX_PRIVATE;
  41         while (spins-- && (!waiters || !*waiters)) {
  42                 if (*addr==val) a_spin();
  43                 else return;
  44         }
  45         if (waiters) a_inc(waiters);
  46         while (*addr==val) {
  47 #ifdef __wasilibc_unmodified_upstream
  48                 __syscall(SYS_futex, addr, FUTEX_WAIT|priv, val, 0) != -ENOSYS
  49                 || __syscall(SYS_futex, addr, FUTEX_WAIT, val, 0);
  50 #else
  51                 __wasilibc_futex_wait(addr, FUTEX_WAIT, val, 0);
  52 #endif
  53         }
  54         if (waiters) a_dec(waiters);
  55 }