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spinlock.c

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+// 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();
+    }
+}
+