From c2258bf4d249c34f26a4ed3c2d6ced81744c654e Mon Sep 17 00:00:00 2001
From: rsc <rsc>
Date: Wed, 28 Nov 2007 20:47:22 +0000
Subject: fork minibug

---
 TRICKS | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

(limited to 'TRICKS')

diff --git a/TRICKS b/TRICKS
index 6883588..b538834 100644
--- a/TRICKS
+++ b/TRICKS
@@ -110,3 +110,27 @@ moves reads down after writes, but the language in
 the spec allows it.  There is no telling whether future
 processors will need it.
 
+---
+
+The code in sys_fork needs to read np->pid before
+setting np->state to RUNNABLE.  
+
+	int
+	sys_fork(void)
+	{
+	  int pid;
+	  struct proc *np;
+	
+	  if((np = copyproc(cp)) == 0)
+	    return -1;
+	  pid = np->pid;
+	  np->state = RUNNABLE;
+	  return pid;
+	}
+
+After setting np->state to RUNNABLE, some other CPU
+might run the process, it might exit, and then it might
+get reused for a different process (with a new pid), all
+before the return statement.  So it's not safe to just do
+"return np->pid;".
+
-- 
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