/**
 * Mutual exclusion spin locks.
 * (Not mutexes as these are spinning locks).
 */

#include "spinlock.h"
#include "riscv.h"
#include "proc.h"
#include "defs.h"

/**
 * The aquire() and release() functions control ownership of the lock.
 * To perform these operations, modern CPU's provide atomic instructions
 * that prevent the cores from stepping on each other's toes, otherwise known
 * as a deadlock.
 *
 * GCC provides a set of built-in functions that allow you to use atomic
 * instructions in an architecture-independent way. These functions are
 * defined in the GCC manual:
 *
 *   See: https://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.html
 *   See: https://en.wikipedia.org/wiki/Memory_barrier
 *
 * On RISC-V, sync_lock_test_and_set turns into an atomic swap:
 *   a5 = 1
 *   s1 = &lk->locked
 *   amoswap.w.aq a5, a5, (s1)
 *
 * On RISC-V, sync_lock_release turns into an atomic swap:
 *   s1 = &lk->locked
 *   amoswap.w zero, zero, (s1)
 *
 *   __sync_synchronize();
 *
 * This function tells the C compiler and the processor to not move loads or stores
 * past this point, to ensure that the critical section's memory
 * references happen strictly after the lock is acquired/locked.
 * On RISC-V, this emits a fence instruction.
 */

/** Initialize spinlock */
void
initlock(struct spinlock *lk, char *name)
{
  lk->name = name;
  lk->locked = 0;
  lk->cpu = 0;
}

/**
 * Acquire the lock.
 * Loops (spins) until the lock is acquired.
 * Panics if the lock is already held by this cpu.
 */
void
acquire(struct spinlock *lk)
{
  push_off(); // disable interrupts to avoid deadlock.

  if(holding(lk)) // If the lock is already held, panic.
    panic("acquire");

  // Spin until aquired. See file header for details
  while(__sync_lock_test_and_set(&lk->locked, 1) != 0) {}
  __sync_synchronize(); // No loads/stores after this point

  // Record info about lock acquisition for holding() and debugging.
  lk->cpu = mycpu();
}

/**
 * Release the lock.
 * Panics if the lock is not held.
 */
void
release(struct spinlock *lk)
{
  if(!holding(lk)) // If the lock is not held, panic.
    panic("release");

  lk->cpu = 0;                      // 0 means unheld
  __sync_synchronize();             // No loads/stores after this point
  __sync_lock_release(&lk->locked); // Essentially lk->locked = 0

  pop_off();
}

// Check whether this cpu is holding the lock.
// Interrupts must be off.
int
holding(struct spinlock *lk)
{
  int r;
  r = (lk->locked && lk->cpu == mycpu());
  return r;
}

// push_off/pop_off are like intr_off()/intr_on() except that they are matched:
// it takes two pop_off()s to undo two push_off()s.  Also, if interrupts
// are initially off, then push_off, pop_off leaves them off.

void
push_off(void)
{
  int old = intr_get();

  intr_off();
  if(mycpu()->noff == 0)
    mycpu()->intena = old;
  mycpu()->noff += 1;
}

void
pop_off(void)
{
  struct cpu *c = mycpu();
  if(intr_get())
    panic("pop_off - interruptible");
  if(c->noff < 1)
    panic("pop_off");
  c->noff -= 1;
  if(c->noff == 0 && c->intena)
    intr_on();
}