From: Robert Morris <rtm@csail.mit.edu>
Date: Thu, 8 Sep 2016 18:45:20 +0000 (-0400)
Subject: use asm() for lock release, not a C assignment
X-Git-Url: https://git.devinivas.org/?a=commitdiff_plain;h=34c2efc1d063f4b366c1017c174d117cc96eb990;p=cs3210-lab0.git

use asm() for lock release, not a C assignment
---

diff --git a/spinlock.c b/spinlock.c
index bf863ef..7b372ef 100644
--- a/spinlock.c
+++ b/spinlock.c
@@ -59,8 +59,10 @@ release(struct spinlock *lk)
   // stores; __sync_synchronize() tells them both to not re-order.
   __sync_synchronize();
 
-  // Release the lock.
-  lk->locked = 0;
+  // Release the lock, equivalent to lk->locked = 0.
+  // This code can't use a C assignment, since it might
+  // not be atomic.
+  asm volatile("movl $0, %0" : "+m" (lk->locked) : );
 
   popcli();
 }