Re: [PATCH 2/2] tracing: Export ftrace API for kernel modules

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 2009-09-15 at 10:01 -0400, Masami Hiramatsu wrote:
> Steven Rostedt wrote:

> > Now I know of two ways to fix this.
> > 
> > 1) The simple way. Up the module ref count so once it registers a
> > function it can never be disabled. Of course there's the "force module
> > unload" but people should not do that anyway.
> > 
> > 2) Create a second hook handler for modules. That is the function caller
> > for modules will go to a wrapper first. This wrapper could disable
> > interrupts or grab a lock or something that would prevent a module from
> > being unloaded as the hooks are being called. Perhaps even disabling
> > preemption while calling the hooks will be enough (this is not something
> > I want the normal function caller to do).
> 
> I think this is better solution.
> Out of curiously, is disabling preemption so harmful?

Yes ;-)

I don't want to disable preemption when I don't have to. The function
tracer that is called can. But actually, it's ever more that that. If
you only register a single function, it will call that function
directly. Then there will always be a race window between when the
function gets called and disabling preemption, even if the called
function disables preemption as the first thing it does.

> 
> > And the current function
> > tracer will optimize that if only one function is registered to mcount,
> > then the mcount caller will call that function directly.
> 
> Would you mean that current mcount hook supports only one handler?

Yes, currently a branch can only got to a single function ;-)

What ftrace does, is if you register a single function, the mcount
location will call that function directly. But if you register an second
function, it changes the mcount call site to call a wrapper that loops
to through all the functions that are registered.

Thus a module version would need to call that wrapper every time. Even
if only one function is called. And this wrapper would have to disable
preemption for the entire loop.


> 
> > 
> > It will still need to up the mod ref count when a probe is added, but it
> > can also remove it.
> > 
> > 
> > The problem with the current method, is that a probe can be executing at
> > anytime. Here's an example if we did it your way.
> > 
> > 1. module installed
> > 2. module adds probe
> > 3. function X in kernel calls probe but gets preempted.
> > 4. module removes probe
> > 5. module unistalled
> > 6. function X in kernel continues to run probe but probe no longer
> > exists --- Oops!
> 
> Agreed, if mcount doesn't disable preemption, this will happen.

And it does not.

-- Steve