Re: [RFC][PATCH] s390: add arch_change_pid for arch updates after task pid change

[Martin Schwidefsky <schwidefsky@de.ibm.com>]

On Thu, 19 Apr 2018 16:07:29 +0200
Oleg Nesterov <oleg@redhat.com> wrote:

> On 04/19, Martin Schwidefsky wrote:
> >
> > In a multi-threaded program any thread can call execve(). If this
> > is not done by the thread group leader, the de_thread() function
> > replaces the pid of the task that calls execve() with the pid of
> > thread group leader. If the task reaches user space again without
> > going over __switch_to() the sampling tag is still set to the old
> > pid.  
> 
> If this is the only reason for arch_change_pid() hook, then perhaps
> it would be better/simpler to add it into de_thread() right after
> change_pid(tsk, PIDTYPE_PID, task_pid(leader)) ?
> 
> note also that this way arch_change_pid() doesn't need any checks and
> any arguments, you can simply do
> 
> void arch_change_pid(void)
> {
> 	S390_lowcore.current_pid = current->pid;
> 	if (test_facility(40))
> 		lpp(&S390_lowcore.lpp);
> }

Yeah, the first patch I created to verify that this indeed the problem
basically looked like your proposal. But then why not make it a more
general hook for any kind of PID change? Dunno if this is a worthwhile
approach, the simpler version for sure works as well.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.