xv6/spinlock.c

// Mutual exclusion spin locks.

#include "types.h"
#include "defs.h"
#include "param.h"
#include "x86.h"
#include "memlayout.h"
#include "mmu.h"
#include "proc.h"
#include "spinlock.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) {
    pushcli(); // disable interrupts to avoid deadlock.
    if (holding(lk)) {
        panic("acquire");
    }

    // The xchg is atomic.
    while (xchg(&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 debugging.
    lk->cpu = mycpu();
    getcallerpcs(&lk, lk->pcs);
}

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

    lk->pcs[0] = 0;
    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) :);

    popcli();
}

// Record the current call stack in pcs[] by following the %ebp chain.
void getcallerpcs(void *v, uint pcs[]) {
    uint *ebp;
    int i;

    ebp = (uint*)v - 2;
    for (i = 0; i < 10; i++) {
        if (ebp == 0 || ebp < (uint*)KERNBASE || ebp == (uint*)0xffffffff) {
            break;
        }
        pcs[i] = ebp[1];     // saved %eip
        ebp = (uint*)ebp[0]; // saved %ebp
    }
    for (; i < 10; i++) {
        pcs[i] = 0;
    }
}

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


// Pushcli/popcli are like cli/sti except that they are matched:
// it takes two popcli to undo two pushcli.  Also, if interrupts
// are off, then pushcli, popcli leaves them off.

void pushcli(void)      {
    int eflags;

    eflags = readeflags();
    cli();
    if (mycpu()->ncli == 0) {
        mycpu()->intena = eflags & FL_IF;
    }
    mycpu()->ncli += 1;
}

void popcli(void)      {
    if (readeflags() & FL_IF) {
        panic("popcli - interruptible");
    }
    if (--mycpu()->ncli < 0) {
        panic("popcli");
    }
    if (mycpu()->ncli == 0 && mycpu()->intena) {
        sti();
    }
}