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Date: Thu, 19 Apr 2018 16:07:29 +0200
From: Oleg Nesterov <oleg@redhat.com>
Subject: Re: [RFC][PATCH] s390: add arch_change_pid for arch updates after
 task pid change
Message-ID: <20180419140729.GA19917@redhat.com>
References: <1524120286-19066-1-git-send-email-schwidefsky@de.ibm.com>
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To: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: linux-arch@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>

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);
}

Oleg.