xv6/spinlock.c
rsc 9fd9f80431 Re: why cpuid() in locking code?
rtm wrote:
> Why does acquire() call cpuid()? Why does release() call cpuid()?

The cpuid in acquire is redundant with the cmpxchg, as you said.
I have removed the cpuid from acquire.

The cpuid in release is actually doing something important,
but not on the hardware.  It keeps gcc from reordering the
lock->locked assignment above the other two during optimization.
(Not that current gcc -O2 would choose to do that, but it is allowed to.)
I have replaced the cpuid in release with a "gcc barrier" that
keeps gcc from moving things around but has no hardware effect.

On a related note, I don't think the cpuid in mpmain is necessary,
for the same reason that the cpuid wasn't needed in release.

As to the question of whether

  acquire();
  x = protected;
  release();

might read protected after release(), I still haven't convinced
myself whether it can.  I'll put the cpuid back into release if
we determine that it can.

Russ
2007-09-30 14:30:04 +00:00

116 lines
2.2 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"
extern int use_console_lock;
// Barrier to gcc's instruction reordering.
static void inline gccbarrier(void)
{
asm volatile("" : : : "memory");
}
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");
while(cmpxchg(0, 1, &lock->locked) == 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;
gccbarrier(); // Keep gcc from moving lock->locked = 0 earlier.
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();
}