xv6

commit c2258bf4d249c34f26a4ed3c2d6ced81744c654e
parent 4f06ae0d4252796ee3245ecd83ec5ea105d6a75b
Author: rsc <rsc>
Date:   Wed, 28 Nov 2007 20:47:22 +0000

fork minibug

Diffstat:
M
TRICKS
 | 
24
++++++++++++++++++++++++
M
sysproc.c
 | 
4
+++-

2 files changed, 27 insertions(+), 1 deletion(-)
diff --git 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;".
+
diff --git a/sysproc.c b/sysproc.c
@@ -7,12 +7,14 @@
 int
 sys_fork(void)
 {
+  int pid;
   struct proc *np;
 
   if((np = copyproc(cp)) == 0)
     return -1;
+  pid = np->pid;
   np->state = RUNNABLE;
-  return np->pid;
+  return pid;
 }
 
 int