Re: [PATCH v3 13/16] exit: Factor thread_group_exited out of pidfd_poll

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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



[Index of Archives]     [Linux Ext4 Filesystem]     [Union Filesystem]     [Filesystem Testing]     [Ceph Users]     [Ecryptfs]     [AutoFS]     [Kernel Newbies]     [Share Photos]     [Security]     [Netfilter]     [Bugtraq]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux Cachefs]     [Reiser Filesystem]     [Linux RAID]     [Samba]     [Device Mapper]     [CEPH Development]

  Powered by Linux