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

On Tue, Aug 4, 2015 at 6:02 PM, Konstantin Belousov <kostikbel_at_gmail.com> wrote:
> On Tue, Aug 04, 2015 at 05:32:28PM +0200, Luigi Rizzo wrote:
>> On Tue, Aug 4, 2015 at 4:53 PM, Konstantin Belousov <kostikbel_at_gmail.com> wrote:
>> > 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 ?
> No.  I probably having troubles answering this exact question.  TDP_KTHREAD
> is used as a shortcut in several places, mostly to avoid some specific
> actions that are useless for the real kernel process.  E.g. the amd64
> context switch code avoids setting some CPU registers when switching to
> the kernel thread.  Hacks to not switch address spaces when switching
> to the kthread were already removed.
>
> Overall, claiming that a thread of the normal user process (i.e. process
> which executes in userspace) is kernel thread is fragile and, if not
> breaking in your tests, could break later.
>>
>> This specific thread is created within a system call by invoking kthread_add()
>> and associated with the user process.
> Did you tried the other approach, by creating the thread from userspace,
> and then executing the syscall which would loop ?  This is how things are
> done by e.g. nfsd.

ok will look at the alternative you suggest.

Specifically, our two threads implement the host part of a virtualized network
interface and operate in a way that is very similar to an interrupt handler,
or vhost-net in KVM. In principle, the user process that issues
the system call could be unaware of the existence and number of
those threads; that is why we went for a "kernel thread" created
within the kernel.

thanks again for the explanation

cheers
luigi

>
> Still, if you looping in the kernel, you must call thread_suspend_check()
> periodically.
>>
>> >
>> > 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 ?
> Most likely yes.



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