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// 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"
extern int use_console_lock;
void
initlock(struct spinlock *lock, char *name)
{
lock->name = name;
lock->locked = 0;
lock->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 *lock)
{
pushcli();
if(holding(lock))
panic("acquire");
// The xchg is atomic.
// It also serializes, so that reads after acquire are not
// reordered before it.
while(xchg(&lock->locked, 1) == 1)
;
// 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.
lock->cpu = cpu() + 10;
getcallerpcs(&lock, lock->pcs);
}
// Release the lock.
void
release(struct spinlock *lock)
{
if(!holding(lock))
panic("release");
lock->pcs[0] = 0;
lock->cpu = 0xffffffff;
// The xchg serializes, so that reads before release are
// not reordered after it. (This reordering would be allowed
// by the Intel manuals, but does not happen on current
// Intel processors. The xchg being asm volatile also keeps
// gcc from delaying the above assignments.)
xchg(&lock->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 = read_eflags();
cli();
if(cpus[cpu()].ncli++ == 0)
cpus[cpu()].intena = eflags & FL_IF;
}
void
popcli(void)
{
if(read_eflags()&FL_IF)
panic("popcli - interruptible");
if(--cpus[cpu()].ncli < 0)
panic("popcli");
if(cpus[cpu()].ncli == 0 && cpus[cpu()].intena)
sti();
}
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