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

[ebiederm@xmission.com (Eric W. Biederman)]

Alexei Starovoitov <alexei.starovoitov@gmail.com> 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@xmission.com>
>> ---
>>  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.

> The rest all looks good to me. Tested with and without bpf_preload patches.
> Feel free to create a frozen branch with this set.

Can I have your Tested-by and Acked-by?

> btw I'll be offline starting tomorrow for a week.
> Will catch up with threads afterwards.

Eric