120 lines
2.7 KiB
C
120 lines
2.7 KiB
C
// Mutual exclusion spin locks.
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#include "types.h"
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#include "defs.h"
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#include "param.h"
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#include "x86.h"
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#include "mmu.h"
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#include "proc.h"
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#include "spinlock.h"
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void
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initlock(struct spinlock *lock, char *name)
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{
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lock->name = name;
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lock->locked = 0;
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lock->cpu = 0xffffffff;
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}
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// Acquire the lock.
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// Loops (spins) until the lock is acquired.
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// Holding a lock for a long time may cause
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// other CPUs to waste time spinning to acquire it.
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void
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acquire(struct spinlock *lock)
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{
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pushcli();
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if(holding(lock))
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panic("acquire");
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// The xchg is atomic.
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// It also serializes, so that reads after acquire are not
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// reordered before it.
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while(xchg(&lock->locked, 1) == 1)
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;
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// Record info about lock acquisition for debugging.
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// The +10 is only so that we can tell the difference
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// between forgetting to initialize lock->cpu
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// and holding a lock on cpu 0.
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lock->cpu = cpu() + 10;
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getcallerpcs(&lock, lock->pcs);
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}
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// Release the lock.
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void
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release(struct spinlock *lock)
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{
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if(!holding(lock))
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panic("release");
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lock->pcs[0] = 0;
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lock->cpu = 0xffffffff;
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// The xchg serializes, so that reads before release are
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// not reordered after it. The 1996 PentiumPro manual (Volume 3,
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// 7.2) says reads can be carried out speculatively and in
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// any order, which implies we need to serialize here.
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// But the 2007 Intel 64 Architecture Memory Ordering White
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// Paper says that Intel 64 and IA-32 will not move a load
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// after a store. So lock->locked = 0 would work here.
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// The xchg being asm volatile ensures gcc emits it after
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// the above assignments (and after the critical section).
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xchg(&lock->locked, 0);
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popcli();
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}
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// Record the current call stack in pcs[] by following the %ebp chain.
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void
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getcallerpcs(void *v, uint pcs[])
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{
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uint *ebp;
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int i;
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ebp = (uint*)v - 2;
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for(i = 0; i < 10; i++){
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if(ebp == 0 || ebp == (uint*)0xffffffff)
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break;
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pcs[i] = ebp[1]; // saved %eip
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ebp = (uint*)ebp[0]; // saved %ebp
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}
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for(; i < 10; i++)
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pcs[i] = 0;
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}
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// Check whether this cpu is holding the lock.
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int
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holding(struct spinlock *lock)
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{
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return lock->locked && lock->cpu == cpu() + 10;
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}
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// Pushcli/popcli are like cli/sti except that they are matched:
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// it takes two popcli to undo two pushcli. Also, if interrupts
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// are off, then pushcli, popcli leaves them off.
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void
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pushcli(void)
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{
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int eflags;
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eflags = readeflags();
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cli();
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if(cpus[cpu()].ncli++ == 0)
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cpus[cpu()].intena = eflags & FL_IF;
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}
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void
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popcli(void)
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{
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if(readeflags()&FL_IF)
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panic("popcli - interruptible");
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if(--cpus[cpu()].ncli < 0)
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panic("popcli");
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if(cpus[cpu()].ncli == 0 && cpus[cpu()].intena)
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sti();
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}
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