Re: [PATCH v3 13/16] exit: Factor thread_group_exited out of pidfd_poll
From: Daniel Borkmann
Date: Tue Jul 07 2020 - 20:05:11 EST
On 7/7/20 7:09 PM, Eric W. Biederman wrote:
Christian Brauner <christian.brauner@xxxxxxxxxx> writes:
On Fri, Jul 03, 2020 at 04:37:47PM -0500, Eric W. Biederman wrote:
Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> writes:
On Thu, Jul 02, 2020 at 11:41:37AM -0500, Eric W. Biederman wrote:
Create an independent helper thread_group_exited report return true
when all threads have passed exit_notify in do_exit. AKA all of the
threads are at least zombies and might be dead or completely gone.
Create this helper by taking the logic out of pidfd_poll where
it is already tested, and adding a missing READ_ONCE on
the read of task->exit_state.
I will be changing the user mode driver code to use this same logic
to know when a user mode driver needs to be restarted.
Place the new helper thread_group_exited in kernel/exit.c and
EXPORT it so it can be used by modules.
Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
---
include/linux/sched/signal.h | 2 ++
kernel/exit.c | 24 ++++++++++++++++++++++++
kernel/fork.c | 6 +-----
3 files changed, 27 insertions(+), 5 deletions(-)
diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 0ee5e696c5d8..1bad18a1d8ba 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -674,6 +674,8 @@ static inline int thread_group_empty(struct task_struct *p)
#define delay_group_leader(p) \
(thread_group_leader(p) && !thread_group_empty(p))
+extern bool thread_group_exited(struct pid *pid);
+
extern struct sighand_struct *__lock_task_sighand(struct task_struct *task,
unsigned long *flags);
diff --git a/kernel/exit.c b/kernel/exit.c
index d3294b611df1..a7f112feb0f6 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1713,6 +1713,30 @@ COMPAT_SYSCALL_DEFINE5(waitid,
}
#endif
+/**
+ * thread_group_exited - check that a thread group has exited
+ * @pid: tgid of thread group to be checked.
+ *
+ * Test if thread group is has exited (all threads are zombies, dead
+ * or completely gone).
+ *
+ * Return: true if the thread group has exited. false otherwise.
+ */
+bool thread_group_exited(struct pid *pid)
+{
+ struct task_struct *task;
+ bool exited;
+
+ rcu_read_lock();
+ task = pid_task(pid, PIDTYPE_PID);
+ exited = !task ||
+ (READ_ONCE(task->exit_state) && thread_group_empty(task));
+ rcu_read_unlock();
+
+ return exited;
+}
I'm not sure why you think READ_ONCE was missing.
It's different in wait_consider_task() where READ_ONCE is needed because
of multiple checks. Here it's done once.
In practice it probably has no effect on the generated code. But
READ_ONCE is about telling the compiler not to be clever. Don't use
tearing loads or stores etc. When all of the other readers are using
READ_ONCE I just get nervous if we have a case that doesn't.
That's not true. The only place where READ_ONCE(->exit_state) is used is
in wait_consider_task() and nowhere else. We had that discussion a while
ago where I or someone proposed to simply place a READ_ONCE() around all
accesses to exit_state for the sake of kcsan and we agreed that it's
unnecessary and not to do this.
But it obviously doesn't hurt to have it.
There is a larger discussion to be had around the proper handling of
exit_state.
In this particular case because we are accessing exit_state with
only rcu_read_lock protection, because the outcome of the read
is about correctness, and because the compiler has nothing else
telling it not to re-read exit_state, I believe we actually need
the READ_ONCE.
At the same time it would take a pretty special compiler to want to
reaccess that field in thread_group_exited.
I have looked through and I don't find any of the other access of
exit_state where the result is about correctness (so that we care)
and we don't hold tasklist_lock.
But I have removed the necessary wording from the commit comment.
Hey Eric, are you planning to push the final version into a topic branch
so it can be pulled into bpf-next as discussed earlier?
Thanks,
Daniel