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

Re: why cpuid() in locking code?
9fd9f804 · rsc · c840f3ec · 9fd9f804 · 9fd9f804 · 9fd9f804
--- a/main.c
+++ b/main.c
@@ -50,7 +50,6 @@ mpmain(void)
  if(cpu() != mp_bcpu())
    lapic_init(cpu());
  setupsegs(0);
-  cpuid(0, 0, 0, 0, 0);  // memory barrier
  cpus[cpu()].booted = 1;
  scheduler();

--- a/spinlock.c
+++ b/spinlock.c
@@ -10,6 +10,12 @@
 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)
 {
@@ -32,10 +38,6 @@ acquire(struct spinlock *lock)
  while(cmpxchg(0, 1, &lock->locked) == 1)
    ;
-  // Serialize instructions: now that lock is acquired, make sure 
-  // we wait for all pending writes from other processors.
-  cpuid(0, 0, 0, 0, 0);  // memory barrier (see Ch 7, IA-32 manual vol 3)
  // Record info about lock acquisition for debugging.
  // The +10 is only so that we can tell the difference
  // between forgetting to initialize lock->cpu
@@ -54,11 +56,9 @@ release(struct spinlock *lock)
  lock->pcs[0] = 0;
  lock->cpu = 0xffffffff;
-  // Serialize instructions: before unlocking the lock, make sure
+  gccbarrier();  // Keep gcc from moving lock->locked = 0 earlier.
-  // to flush any pending memory writes from this processor.
-  cpuid(0, 0, 0, 0, 0);  // memory barrier (see Ch 7, IA-32 manual vol 3)
  lock->locked = 0;
  popcli();
 }

--- a/x86.h
+++ b/x86.h
@@ -96,6 +96,7 @@ write_eflags(uint eflags)
  asm volatile("pushl %0; popfl" : : "r" (eflags));
 }
+// XXX: Kill this if not used.
 static inline void
 cpuid(uint info, uint *eaxp, uint *ebxp, uint *ecxp, uint *edxp)
 {