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

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Message-ID: <alpine.DEB.2.00.0908261040440.23885@gandalf.stny.rr.com>
Date:	Wed, 26 Aug 2009 10:41:38 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Frederic Weisbecker <fweisbec@...il.com>,
	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>,
	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, 26 Aug 2009, Peter Zijlstra wrote:

> On Wed, 2009-08-26 at 09:48 -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.
> 
> Weren't there a few sites in the kernel where kernel threads temporarily
> borrow the mm from someone?

All kernel threads borrow a mm, but they don't set it as their own. That's 
what t->active_mm is for.

-- Steve

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