spinlock.c


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// Mutual exclusion spin locks.

#include "types.h"
#include "param.h"
#include "memlayout.h"
#include "spinlock.h"
#include "riscv.h"
#include "proc.h"
#include "defs.h"

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.
// Holding a lock for a long time may cause
// other CPUs to waste time spinning to acquire it.
void
acquire(struct spinlock *lk)
{
  push_off(); // disable interrupts to avoid deadlock.
  if(holding(lk))
    panic("acquire");

  // The xchg is atomic.
  //while(xchg(&lk->locked, 1) != 0)
  //  ;
  while(__sync_lock_test_and_set(&lk->locked, 1) != 0)
    ;

  // Tell 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 after the lock is acquired.
  __sync_synchronize();

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

// Release the lock.
void
release(struct spinlock *lk)
{
  if(!holding(lk))
    panic("release");

  lk->cpu = 0;

  // Tell the C compiler and the processor to not move loads or stores
  // past this point, to ensure that all the stores in the critical
  // section are visible to other cores before the lock is released.
  // Both the C compiler and the hardware may re-order loads and
  // stores; __sync_synchronize() tells them both not to.
  __sync_synchronize();

  // Release the lock, equivalent to lk->locked = 0.
  // This code can't use a C assignment, since it might
  // not be atomic. A real OS would use C atomics here.
  //asm volatile("movl $0, %0" : "+m" (lk->locked) : );
  __sync_lock_release(&lk->locked);

  pop_off();
}

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

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

void
push_off(void)
{
  struct cpu *c = mycpu();
  int old = intr_get();

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

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