mop3/kernel/sync/rw_spin_lock.c

#include <libk/assert.h>
#include <libk/std.h>
#include <sync/rw_spin_lock.h>
#include <sys/debug.h>
#include <sys/spin_lock.h>

#define WRITER_WAIT (1U << 31)
#define READER_MASK (~WRITER_WAIT)

void rw_spin_read_lock (rw_spin_lock_t* rw) {
  uint32_t value;

  for (;;) {
    value = atomic_load_explicit (rw, memory_order_relaxed);

    if ((value & WRITER_WAIT) == 0) {
      if (atomic_compare_exchange_weak_explicit (rw, &value, value + 1, memory_order_acquire,
                                                 memory_order_relaxed)) {
        return;
      }
    }

    spin_lock_relax ();
  }
}

void rw_spin_read_unlock (rw_spin_lock_t* rw) {
  uint32_t old = atomic_fetch_sub_explicit (rw, 1, memory_order_release);
  assert ((old & READER_MASK) > 0);
}

void rw_spin_write_lock (rw_spin_lock_t* rw) {
  uint32_t value;

  /* announce writer */
  for (;;) {
    value = atomic_load_explicit (rw, memory_order_relaxed);

    if ((value & WRITER_WAIT) == 0) {
      if (atomic_compare_exchange_weak_explicit (rw, &value, (value | WRITER_WAIT),
                                                 memory_order_acquire, memory_order_relaxed))
        break;
    } else
      spin_lock_relax ();
  }

  /* wait for readers */
  for (;;) {
    value = atomic_load_explicit (rw, memory_order_acquire);
    if ((value & READER_MASK) == 0)
      return;

    spin_lock_relax ();
  }
}

void rw_spin_write_unlock (rw_spin_lock_t* rw) {
  atomic_store_explicit (rw, 0, memory_order_release);
}