9fd9f80431
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
117 lines
2.2 KiB
C
117 lines
2.2 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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extern int use_console_lock;
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// Barrier to gcc's instruction reordering.
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static void inline gccbarrier(void)
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{
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asm volatile("" : : : "memory");
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}
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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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while(cmpxchg(0, 1, &lock->locked) == 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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gccbarrier(); // Keep gcc from moving lock->locked = 0 earlier.
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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 = read_eflags();
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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(read_eflags()&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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