linux-kernel - Re: [PATCH 08/12] add trace events for each syscall entry/exit

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Message-ID: <20090826135331.GF6009@nowhere>
Date:	Wed, 26 Aug 2009 15:53:35 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>,
	Jason Baron <jbaron@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	jiayingz@...gle.com, mbligh@...gle.com, lizf@...fujitsu.com,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH 08/12] add trace events for each syscall entry/exit

On Wed, Aug 26, 2009 at 09:48:48AM -0400, Steven Rostedt wrote:
> 
> On Wed, 26 Aug 2009, Frederic Weisbecker wrote:
> > > 
> > > That won't always work as pointed out in the other example:
> > > - Process doing sys_init_module then scheduled away
> > > - User enables syscall tracing -> TIF_SYSCALL_FTRACE gets set
> > > - init function of the module gets called and is doing kernel_thread()
> > >   (old API) -> kernel thread inherits TIF_SYSCALL_FTRACE.
> > > 
> > > I don't think that's what you want. You might want to clear the flag for
> > > new processes during fork (only for kernel threads I would guess).
> > > 
> > > At least the current patch leaves a hole.
> > 
> > 
> > Ah, there are callsites that use kernel_thread() directly?
> > Does it means that t->mm could be non NULL for such resulting
> > kernel threads, in that case it would be hard to hook on
> > do_fork() to check that.
> 
> All kernel threads have a NULL t->mm. Since do_fork is called by kthreadd 
> and not by kthread_create, the caller of do_fork will also have a
> t->mm = NULL.
> 
> -- Steve
> 

Yeah, that's the case with kthread_create() creation fashion,
but what if you create a kernel thread using the low level
kernel_thread() directly (ie: without relaying on kthreadd queue)?

Especially in Heiko example, it seems to be a duplication of user
task.

I wonder what obvious think I'm missing here...

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