From e7a5b3c5ee2d440892ab7e9bb98ad82efb25219f Mon Sep 17 00:00:00 2001
From: rsc <rsc>
Date: Thu, 7 Sep 2006 16:53:49 +0000
Subject: [PATCH] comment memory barriers

---
 spinlock.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/spinlock.c b/spinlock.c
index 486c217..2e604b7 100644
--- a/spinlock.c
+++ b/spinlock.c
@@ -50,6 +50,10 @@ acquire(struct spinlock *lock)
 
   while(cmpxchg(0, 1, &lock->locked) == 1)
     ;
+
+  // Now that lock is acquired, make sure 
+  // we wait for all pending writes from other
+  // processors.
   cpuid(0, 0, 0, 0, 0);  // memory barrier
   
   // Record info about lock acquisition for debugging.
@@ -64,13 +68,16 @@ acquire(struct spinlock *lock)
 void
 release(struct spinlock *lock)
 {
-
   if(!holding(lock))
     panic("release");
 
   lock->pcs[0] = 0;
   lock->cpu = 0xffffffff;
+  
+  // Before unlocking the lock, make sure to flush
+  // any pending memory writes from this processor.
   cpuid(0, 0, 0, 0, 0);  // memory barrier
+
   lock->locked = 0;
   if(--cpus[cpu()].nlock == 0)
     sti();