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Message-ID: <20140220173902.GA21853@redhat.com>
Date: Thu, 20 Feb 2014 18:39:02 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Al Viro <viro@...IV.linux.org.uk>,
Jan Kratochvil <jan.kratochvil@...hat.com>,
Lennart Poettering <lpoetter@...hat.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Michal Schmidt <mschmidt@...hat.com>,
Roland McGrath <roland@...k.frob.com>,
Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: [PATCH 2/5] wait: introduce EXIT_TRACE to avoid the racy
EXIT_DEAD->EXIT_ZOMBIE transition
wait_task_zombie() first does EXIT_ZOMBIE->EXIT_DEAD transition and
drops tasklist_lock. If this task is not the natural child and it is
traced, we change its state back to EXIT_ZOMBIE for ->real_parent.
The last transition is racy, this is even documented in 50b8d257486a
"ptrace: partially fix the do_wait(WEXITED) vs EXIT_DEAD->EXIT_ZOMBIE
race". wait_consider_task() tries to detect this transition and clear
->notask_error but we can't rely on ptrace_reparented(), debugger can
exit and do ptrace_unlink() before its sub-thread sets EXIT_ZOMBIE.
And there is another problem which were missed before: this transition
can also race with reparent_leader() which doesn't reset >exit_signal
if EXIT_DEAD, assuming that this task must be reaped by someone else.
So the tracee can be re-parented with ->exit_signal != SIGCHLD, and if
/sbin/init doesn't use __WALL it becomes unreapable. This was fixed by
the previous commit, but it was the temporary hack.
1. Add the new exit_state, EXIT_TRACE. It means that the task is the
traced zombie, debugger is going to detach and notify its natural
parent.
This new state is actually EXIT_ZOMBIE | EXIT_DEAD. This way we
can avoid the changes in proc/kgdb code, get_task_state() still
reports "X (dead)" in this case.
Note: with or without this change userspace can see Z -> X -> Z
transition. Not really bad, but probably makes sense to fix.
2. Change wait_task_zombie() to use EXIT_TRACE instead of EXIT_DEAD
if we need to notify the ->real_parent.
3. Revert the previous hack in reparent_leader(), now that EXIT_DEAD
is always the final state we can safely ignore such a task.
4. Change wait_consider_task() to check EXIT_TRACE separately and kill
the racy and no longer needed ptrace_reparented() case.
If ptrace == T an EXIT_TRACE thread should be simply ignored, the
owner of this state is going to ptrace_unlink() this task. We can
pretend that it was already removed from ->ptraced list.
Otherwise we should skip this thread too but clear ->notask_error,
we must be the natural parent and debugger is going to untrace and
notify us. IOW, this doesn't differ from "EXIT_ZOMBIE && p->ptrace"
even if the task was already untraced.
Reported-by: Jan Kratochvil <jan.kratochvil@...hat.com>
Reported-and-tested-by: Michal Schmidt <mschmidt@...hat.com>
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
include/linux/sched.h | 1 +
kernel/exit.c | 50 ++++++++++++++++++++----------------------------
2 files changed, 22 insertions(+), 29 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index a781dec..b3593ac 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -206,6 +206,7 @@ print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq);
/* in tsk->exit_state */
#define EXIT_ZOMBIE 16
#define EXIT_DEAD 32
+#define EXIT_TRACE (EXIT_ZOMBIE | EXIT_DEAD)
/* in tsk->state again */
#define TASK_DEAD 64
#define TASK_WAKEKILL 128
diff --git a/kernel/exit.c b/kernel/exit.c
index 5281522..c702824 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -560,6 +560,9 @@ static void reparent_leader(struct task_struct *father, struct task_struct *p,
struct list_head *dead)
{
list_move_tail(&p->sibling, &p->real_parent->children);
+
+ if (p->exit_state == EXIT_DEAD)
+ return;
/*
* If this is a threaded reparent there is no need to
* notify anyone anything has happened.
@@ -567,19 +570,9 @@ static void reparent_leader(struct task_struct *father, struct task_struct *p,
if (same_thread_group(p->real_parent, father))
return;
- /*
- * We don't want people slaying init.
- *
- * Note: we do this even if it is EXIT_DEAD, wait_task_zombie()
- * can change ->exit_state to EXIT_ZOMBIE. If this is the final
- * state, do_notify_parent() was already called and ->exit_signal
- * doesn't matter.
- */
+ /* We don't want people slaying init. */
p->exit_signal = SIGCHLD;
- if (p->exit_state == EXIT_DEAD)
- return;
-
/* If it has exited notify the new parent about this child's death. */
if (!p->ptrace &&
p->exit_state == EXIT_ZOMBIE && thread_group_empty(p)) {
@@ -1045,17 +1038,13 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
return wait_noreap_copyout(wo, p, pid, uid, why, status);
}
+ traced = ptrace_reparented(p);
/*
- * Try to move the task's state to DEAD
- * only one thread is allowed to do this:
+ * Move the task's state to DEAD/TRACE, only one thread can do this.
*/
- state = xchg(&p->exit_state, EXIT_DEAD);
- if (state != EXIT_ZOMBIE) {
- BUG_ON(state != EXIT_DEAD);
+ state = traced ? EXIT_TRACE : EXIT_DEAD;
+ if (cmpxchg(&p->exit_state, EXIT_ZOMBIE, state) != EXIT_ZOMBIE)
return 0;
- }
-
- traced = ptrace_reparented(p);
/*
* It can be ptraced but not reparented, check
* thread_group_leader() to filter out sub-threads.
@@ -1116,7 +1105,7 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
/*
* Now we are sure this task is interesting, and no other
- * thread can reap it because we set its state to EXIT_DEAD.
+ * thread can reap it because we its state == DEAD/TRACE.
*/
read_unlock(&tasklist_lock);
@@ -1161,14 +1150,14 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
* If this is not a sub-thread, notify the parent.
* If parent wants a zombie, don't release it now.
*/
+ state = EXIT_DEAD;
if (thread_group_leader(p) &&
- !do_notify_parent(p, p->exit_signal)) {
- p->exit_state = EXIT_ZOMBIE;
- p = NULL;
- }
+ !do_notify_parent(p, p->exit_signal))
+ state = EXIT_ZOMBIE;
+ p->exit_state = state;
write_unlock_irq(&tasklist_lock);
}
- if (p != NULL)
+ if (state == EXIT_DEAD)
release_task(p);
return retval;
@@ -1364,12 +1353,15 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace,
}
/* dead body doesn't have much to contribute */
- if (unlikely(p->exit_state == EXIT_DEAD)) {
+ if (unlikely(p->exit_state == EXIT_DEAD))
+ return 0;
+
+ if (unlikely(p->exit_state == EXIT_TRACE)) {
/*
- * But do not ignore this task until the tracer does
- * wait_task_zombie()->do_notify_parent().
+ * ptrace == 0 means we are the natural parent. In this case
+ * we should clear notask_error, debugger will notify us.
*/
- if (likely(!ptrace) && unlikely(ptrace_reparented(p)))
+ if (likely(!ptrace))
wo->notask_error = 0;
return 0;
}
--
1.5.5.1
--
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