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

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Message-ID: <20090826141052.GB5946@osiris.boeblingen.de.ibm.com>
Date:	Wed, 26 Aug 2009 16:10:52 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>,
	Jason Baron <jbaron@...hat.com>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, laijs@...fujitsu.com, rostedt@...dmis.org,
	peterz@...radead.org, 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 03:30:22PM +0200, Frederic Weisbecker wrote:
> On Wed, Aug 26, 2009 at 02:59:43PM +0200, Heiko Carstens wrote:
> > > Yeah, and as told before, syscalls tracing from kernel thread is
> > > an interesting point but we can't do it that way.
> > > 
> > > I'm queuing this patch for .32, but I need you Signed-off-by to apply it :)
> > 
> > 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.

Oh yes, you are right. kernel threads created with kernel_thread()
have t->mm != NULL if the forking process has an mm too.

There are very few callsites left which still use kernel_thread().
(the last one in s390 driver code will be gone after the next
merge window).

As far as I can there are only four callsites left
(excluding staging): jffs2 and three in net/bluetooth/*
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