[PATCH -mm] utrace: fix double free re __rcu_process_callbacks()

[Alexey Dobriyan <adobriyan@sw.ru>]

The following patch fixes double free manifesting itself as crash in
__rcu_process_callbasks():
http://marc.info/?l=linux-kernel&m=117518764517017&w=2
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=229112

The problem is with check_dead_utrace() conditionally scheduling
"struct utrace" for freeing but not cleaning struct task_struct::utrace
pointer leaving it reachable:

	tsk->utrace_flags = flags;
	if (flags)
		spin_unlock(&utrace->lock);
	else
		rcu_utrace_free(utrace);

OTOH, utrace_release_task() first clears ->utrace pointer, then frees
struct utrace itself:

Roland inserted some debugging into 2.6.21-rc6-mm1 so that aforementined
double free couldn't be reproduced without seeing
BUG at kernel/utrace.c:176 first. It triggers if one struct utrace were
passed to rcu_utrace_free() second time.

With patch applied I no longer see¹ BUG message and double frees on
2-way P3, 8-way ia64, Core 2 Duo boxes. Testcase is at the first link.

I _think_ it adds leak if utrace_reap() takes branch without freeing
but, well, I hope Roland will give me some clue on how to fix it too.

Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
---

 kernel/utrace.c |    6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

¹ But I see whole can of other bugs! I think they were already lurking
  but weren't easily reproducable without hitting double-free first.
  FWIW, it's
  	BUG_ON(!list_empty(&tsk->ptracees));
	oops at the beginning of remove_engine()
	NULL ->report_quiesce call which is absent in ptrace utrace ops
	BUG_ON(tracehook_check_released(p));

--- a/kernel/utrace.c
+++ b/kernel/utrace.c
@@ -205,7 +205,6 @@ utrace_clear_tsk(struct task_struct *tsk
 	if (utrace->u.live.signal == NULL) {
 		task_lock(tsk);
 		if (likely(tsk->utrace != NULL)) {
-			rcu_assign_pointer(tsk->utrace, NULL);
 			tsk->utrace_flags &= UTRACE_ACTION_NOREAP;
 		}
 		task_unlock(tsk);
@@ -305,10 +304,7 @@ check_dead_utrace(struct task_struct *ts
 	}
 
 	tsk->utrace_flags = flags;
-	if (flags)
-		spin_unlock(&utrace->lock);
-	else
-		rcu_utrace_free(utrace);
+	spin_unlock(&utrace->lock);
 
 	/*
 	 * Now we're finished updating the utrace state.