From 34c2efc1d063f4b366c1017c174d117cc96eb990 Mon Sep 17 00:00:00 2001
From: Robert Morris <rtm@csail.mit.edu>
Date: Thu, 8 Sep 2016 14:45:20 -0400
Subject: [PATCH] use asm() for lock release, not a C assignment

---
 spinlock.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/spinlock.c b/spinlock.c
index bf863ef104..7b372ef3f1 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();
 }