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

[Christian Brauner]

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.

Christian