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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();
}
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