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SubjectRe: module-placed markers/tracepoints
* Frank Ch. Eigler (fche@redhat.com) wrote:
> Hi -
>
> Some locals are wondering -- is there code for (or need for new code
> for) incrementing module reference counts while markers and/or
> tracepoints resident in modules have active clients?
>
> - FChE

Hi Frank,

Probe module unloading is supposed to be done automatically assuming the
following module unload behavior :

1 - Module unload code takes the stop machine lock.
2 - It is assumed that the module exit function is called at that time,
while no preempt-disabled section is executed. This function
unregisters the callbacks.
3 - Given that preemption is disabled around tracepoint/marker calls, no
calls are left to a freed module.

Actually, I notice that it might not be the case. I took for granted
what (I can't remember who ? Rusty ?) told me about module unload code.
If we look at sys_delete_module() :


/* Stop the machine so refcounts can't move and disable module. */
ret = try_stop_module(mod, flags, &forced);
if (ret != 0)
goto out;

/* Never wait if forced. */
if (!forced && module_refcount(mod) != 0)
wait_for_zero_refcount(mod);

mutex_unlock(&module_mutex);
/* Final destruction now noone is using it. */
if (mod->exit != NULL)
mod->exit();

So we see that mod->exit(), which I believe is responsible for calling
the module exit() functions, is called after the stop_machine, not
during the stop machine.

Therefore, what would be needed here is to add a synchronize_sched()
after mod->exit() in module.c or after each tracepoint/marker unregister
in marker.c/tracepoint.c. However, I'd really prefer to add this to
module.c since adding this for _every_ unregister will slow down
processing or multiple probe unregistration too much.

Rusty, am I understanding that correctly ?

Mathieu


--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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