Re: [uml-devel] Re: UML fills /tmp irreversibly (2.6.9-bb4 on SuSE 9.2)

[Gerd Knorr <kraxel@bytesex.org>]

> Roland McGrath:
> 
>   o fix bogus ECHILD return from wait* with zombie group leader
> 
> I'm not sure this is the fix, but it is possible indeed, given that the 
> problem UML triggered was, IIRC, that when it exited, there wasn't a proper 
> cleanup of the status, and the process became invisible but still kept a 
> reference to the file in /tmp, preventing it from being deleted...

Yep, that makes sense.  The race thing also explains why it doesn't
allways happen.  I still can't reproduce it on my machine btw, so it's
hard for me to test whenever that really fixes it or not.

> I think you can search for it on linux.bkbits.net... I'll do that when I have 
> time...

Attached below for reference.

  Gerd

# This is a BitKeeper generated diff -Nru style patch.
#
# ChangeSet
#   2004/12/17 09:18:41-08:00 roland@redhat.com 
#   [PATCH] fix bogus ECHILD return from wait* with zombie group leader
#   
#   Klaus Dittrich observed this bug and posted a test case for it.
#   
#   This patch fixes both that failure mode and some others possible.  What
#   Klaus saw was a false negative (i.e.  ECHILD when there was a child)
#   when the group leader was a zombie but delayed because other children
#   live; in the test program this happens in a race between the two threads
#   dying on a signal.
#   
#   The change to the TASK_TRACED case avoids a potential false positive
#   (blocking, or WNOHANG returning 0, when there are really no children
#   left), in the race condition where my_ptrace_child returns zero.
#   
#   Signed-off-by: Roland McGrath <roland@redhat.com>
#   Signed-off-by: Andrew Morton <akpm@osdl.org>
#   Signed-off-by: Linus Torvalds <torvalds@osdl.org>
# 
# kernel/exit.c
#   2004/12/17 00:09:08-08:00 roland@redhat.com +13 -2
#   fix bogus ECHILD return from wait* with zombie group leader
# 
diff -Nru a/kernel/exit.c b/kernel/exit.c
--- a/kernel/exit.c	2005-01-10 14:00:38 +01:00
+++ b/kernel/exit.c	2005-01-10 14:00:38 +01:00
@@ -1319,6 +1319,10 @@
 
 	add_wait_queue(&current->wait_chldexit,&wait);
 repeat:
+	/*
+	 * We will set this flag if we see any child that might later
+	 * match our criteria, even if we are not able to reap it yet.
+	 */
 	flag = 0;
 	current->state = TASK_INTERRUPTIBLE;
 	read_lock(&tasklist_lock);
@@ -1337,11 +1341,14 @@
 
 			switch (p->state) {
 			case TASK_TRACED:
-				flag = 1;
 				if (!my_ptrace_child(p))
 					continue;
 				/*FALLTHROUGH*/
 			case TASK_STOPPED:
+				/*
+				 * It's stopped now, so it might later
+				 * continue, exit, or stop again.
+				 */
 				flag = 1;
 				if (!(options & WUNTRACED) &&
 				    !my_ptrace_child(p))
@@ -1377,8 +1384,12 @@
 						goto end;
 					break;
 				}
-				flag = 1;
 check_continued:
+				/*
+				 * It's running now, so it might later
+				 * exit, stop, or stop and then continue.
+				 */
+				flag = 1;
 				if (!unlikely(options & WCONTINUED))
 					continue;
 				retval = wait_task_continued(


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