xv6-65oo2/spinlock.c
2009-07-11 19:26:51 -07:00

120 lines
2.6 KiB
C

// Mutual exclusion spin locks.
#include "types.h"
#include "defs.h"
#include "param.h"
#include "x86.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 = 0xffffffff;
}
// 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();
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.
// The +10 is only so that we can tell the difference
// between forgetting to initialize lock->cpu
// and holding a lock on cpu 0.
lk->cpu = cpu() + 10;
getcallerpcs(&lk, lk->pcs);
}
// Release the lock.
void
release(struct spinlock *lk)
{
if(!holding(lk))
panic("release");
lk->pcs[0] = 0;
lk->cpu = 0xffffffff;
// 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*)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() + 10;
}
// 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(c->ncli++ == 0)
c->intena = eflags & FL_IF;
}
void
popcli(void)
{
if(readeflags()&FL_IF)
panic("popcli - interruptible");
if(--c->ncli < 0)
panic("popcli");
if(c->ncli == 0 && c->intena)
sti();
}