linux-kernel - Re: [PATCH RESEND 2/2] tracing: syscall_regfunc() should not skip kernel threads

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Message-ID: <20140410190025.GA13925@redhat.com>
Date:	Thu, 10 Apr 2014 21:00:25 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>
Subject: Re: [PATCH RESEND 2/2] tracing: syscall_regfunc() should not skip
	kernel threads

On 04/10, Oleg Nesterov wrote:
>
> On 04/10, Oleg Nesterov wrote:
> >
> > On 04/10, Steven Rostedt wrote:
> > >
> > > Hendrik, we are debating about removing
> > > cc3b13c11c567c69a6356be98d0c03ff11541d5c as it stops
> > > call_usermodehelper tasks from tracing their syscalls.
> > >
> > > If Hendrik has no problems with this, neither do I.
> >
> > OK.
> >
> > cc3b13c11c567 mentions ret_from_fork, today copy_thread(PF_KTHREAD) uses
> > ret_from_kernel_thread on 32bit, and still ret_from_fork on 64 bit but
> > in the last case it checks PF_KTHREAD... I am wondering why they both
> > (ret_from_kernel_thread and "1: " label in ret_from_fork) can't simply
> > call do_exit() at the end?
>
> probably because we need to change all architectures...

Heh. And because "call *%rbx" can actually return if this kernel thread
execs ;)

But in this case we want TIF_SYSCALL_TRACING, so

> > And, since they do not, every kernel_thread's function (fn argument of
> > kernel_thread) must call do_exit itself?
>
> Hmm yes. See fb45550d76bb5 "make sure that kernel_thread() callbacks call
> do_exit() themselves".
>
> > Looks a bit strange, I guess I missed something obvious.
>
> And I forgot to mention, given that the kernel_thread() callback should
> call do_exit() itself, then this part of cc3b13c11c567c69a63
>
> 	one case when a kernel thread can reach the
> 	usual syscall exit tracing path: when we create a kernel thread, the
> 	child comes to ret_from_fork
>
> is no longer relevant? A PF_KTHREAD child should never return from the
> callback and thus it should never do "jmp syscall_exit" ?

this is still true, I guess.

Oleg.

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