[PATCH] Module : call synchronize_sched() between module exit() and free.

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

> Hi Mathieu,
> 
>    Yes: stop_machine is merely used to atomically check the module refcount 
> for zero and set the state so it can't be incremented again (ie. 
> try_module_get will fail).
> 
>    So placing a tracepoint or marker in a module does not bump the module 
> refcount?  If that's true, then there needs to be some kind of 
> remove_markers_from_module() call after module->exit(), which should do the 
> synchronize_sched() or whatever, right?
> 
> Rusty.

Actually, it's not placing a marker/tracepoint in a module which causes
a problem, this is a simple function call after all, and correctly dealt
with by current module.c code.

The problem comes from a probe function (the callback) that would be
registered to be called from a marker and would sit in an unloadable 
kernel module. I would not want to tie the refcount of the probe modules
to the fact that they are connected to a marker because it would then
become impossible to unload them due to the fact that unregistration is
done in module exit().

This is one of the reasons why I disable preemption around the marker
site (the function call) : to make sure I can can unregister the
callback, wait for a quiescent state (with synchronize_sched()) and then
free the module memory.

This would give the following supplementary guarantee about module
teardown : every function called with preemption off and unregistered in
the module exit() would reach a quiescent state before the module is
freed. Given this does apply to rarely used code (module unload), I
think it might be ok to simply add a call to synchronize_sched() before
the module memory is freed. Not tying this to markers/tracepoints would
keep the behavior consistant across various build options, which is IMHO
a good thing.

I could also just document that a mandatory "synchronize_sched()" should
be called at the end of the probe module exit() function which makes
sure the probes has reached a quiescent state.

I don't want to add a synchronize_sched() into the marker/tracepoint
probe unregistration code because I want to keep batch probe
unregistration fast enough so it does no take ~5 seconds to unload ~100
probes. (may take longer on a loaded SMP system)

Mathieu

-- 
Mathieu Desnoyers
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