lkml.org 
[lkml]   [2009]   [Apr]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 1/5] ftrace: use module notifier for function tracer

* Steven Rostedt <rostedt@goodmis.org> wrote:

>
> On Tue, 21 Apr 2009, Rusty Russell wrote:
>
> > On Mon, 20 Apr 2009 11:27:35 pm Steven Rostedt wrote:
> > >
> > > On Sun, 19 Apr 2009, Rusty Russell wrote:
> > >
> > > > On Thu, 16 Apr 2009 11:48:31 am Steven Rostedt wrote:
> > > > > From: Steven Rostedt <srostedt@redhat.com>
> > > > >
> > > > > Impact: fix and clean up
> > > > >
> > > > > The hooks in the module code for the function tracer must be called
> > > > > before any of that module code runs. The function tracer hooks
> > > > > modify the module (replacing calls to mcount to nops). If the code
> > > > > is executed while the change occurs, then the CPU can take a GPF.
> > > > >
> > > > > To handle the above with a bit of paranoia, I originally implemented
> > > > > the hooks as calls directly from the module code.
> > > > >
> > > > > After examining the notifier calls, it looks as though the start up
> > > > > notify is called before any of the module's code is executed. This makes
> > > > > the use of the notify safe with ftrace.
> > > >
> > > > Hi Steven,
> > > >
> > > > Unfortunately not: we do parse_args, which can call into the module code.
> > > > (Though it shouldn't do anything "significant", as it won't get a chance to
> > > > clean up if module load fails later).
> > > >
> > > > I think you need to do something else in general. Share the module_mutex for
> > > > the ftrace code? The ksplice guys have a similar issue, so maybe we should
> > > > generalize this into a "kernel_text" mutex?
> > >
> > > Hi Rusty,
> > >
> > > Thanks, for the update. I think we may still be OK.
> >
> > Agreed, just wanted to make sure you were aware.
> >
> > > Can those parse_args kick off threads? Hmm, probably. Sounds nasty to
> > > me.
> >
> > Not without a bug. Imagine you have a "create_threads" module_param, someone
> > loads the module with two args "create_threads crap". We call the
> > create_threads parse function via parse_args, then hit the crap parameter
> > and free the module. Oops.
> >
> > > The other thing is, if the parse_args code is only in "__init" then they
> > > also will not be touched.
> >
> > It can be non-init for sysfs access.
> >
> > FWIW:
> > Acked-by: Rusty Russell <rusty@rustcorp.com.au>
>
> Thanks Rusty!
>
> Ingo,
>
> This change also fixes a possible deadlock in mainline between the
> ftrace_lock and the module_mutex. Perhaps we should push this to
> Linus?
>
> The possible deadlock is if a user unloads/loads modules at the
> same time starts the function graph tracer. Highly unlikely to
> happen, but it does spit out a lockdep warning.

It impacts the module code so it would be nicer if Rusty pushed it.

To me it's a bit too large:

4 files changed, 75 insertions(+), 45 deletions(-)

and the race too narrow. (has it been reported in practice?)

Ingo


\
 
 \ /
  Last update: 2009-04-21 16:01    [W:0.074 / U:0.228 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site