proper way to terminate a kthread when the parent process dies ?

[Luigi Rizzo]

On Tue, Aug 4, 2015 at 4:53 PM, Konstantin Belousov <kostikbel at gmail.com> wrote:
> On Tue, Aug 04, 2015 at 04:38:14PM +0200, Luigi Rizzo wrote:
>> Hi,
>> we have a doubt on the proper way to terminate a kernel thread that
>> has been associated to a user process U within a system call with
>>
>>         kthread_add( .. , .., p, ... )
>>
>> (p is the struct proc * of the calling process, U)
>>
>> When U terminates and goes into kern_exit.c :: exit1()
>> the kernel thread sees the following conditions:
>>
>>         P_SHOULDSTOP(td->td_proc) is TRUE
>>
>> td->td_flags has TDF_ASTPENDING | TDF_NEEDSUSPCHK set
>>
>> We are not sure what is the proper way to terminate
>> our kernel thread, whose body is the following:
>>
>>         while (must_run) { // someone will set must_run = 0
>>                 <check_for_forced_termination>
>>                 kthread_suspend_check(); // void
>>                 work_or_short_tsleep(); // potentially se
>>         }
>>         kthread_exit();
>>
>> We have seen different ways for the <check_for_forced_termination>
>>
>> 1.      if (P_SHOULDSTOP(td->td_proc)
>>                 break; // kthread_exit() is called outside the loop
>>
>> 2.      if (P_SHOULDSTOP(td->td_proc)
>>                 thread_suspend_check(0); // which then terminates the thread
>>         // this is done in sys/rpc/svc.c
>>
>> We are a bit unsure whether calling the thread_*() function in a kthread
>> is correct -- but there is an example in the kernel.
>>
>> Variants involve locking td->td_proc (but is it necessary ? The process
>> won't go away until all child threads die), or checking the td_tdflags
>> instead of the parent process' flags.
>>
>> So what is the correct way ?
>
> If this is a thread of the normal user process, then it is not a kernel
> thread, even if it never leaves the kernel mode.

thanks for the answer.

i do not really know what is the difference between a "kernel thread"
and a "thread".
Could you clarify what is the distinctive feature between the two ?
Perhaps being owned by pid 0 ?

This specific thread is created within a system call by invoking kthread_add()
and associated with the user process.

>
> You must call thread_suspend_check() in any in-kernel loop to allow the
> stops and process exit to work.

so does it mean that the kthread_suspend_check() is incorrect and we should
use thread_suspend_check() instead ?

cheers
luigi


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