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.
    // It also serializes, so that reads after acquire are not
    // reordered before it.
    while (xchg(&lk->locked, 1) != 0);

    // Record info about lock acquisition for debugging.
    lk->cpu = cpu;
    getcallerpcs(&lk, lk->pcs);
}

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

    lk->pcs[0] = 0;
    lk->cpu = 0;

    // The xchg serializes, so that reads before release are
    // not reordered after it.  The 1996 PentiumPro manual (Volume 3,
    // 7.2) says reads can be carried out speculatively and in
    // any order, which implies we need to serialize here.
    // But the 2007 Intel 64 Architecture Memory Ordering White
    // Paper says that Intel 64 and IA-32 will not move a load
    // after a store. So lock->locked = 0 would work here.
    // The xchg being asm volatile ensures gcc emits it after
    // the above assignments (and after the critical section).
    xchg(&lk->locked, 0);

    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) {
    return lock->locked && lock->cpu == cpu;
}


// 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 (cpu->ncli++ == 0)
        cpu->intena = eflags & FL_IF;
}

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