linux-kernel - Re: module-placed markers/tracepoints

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Date:	Tue, 29 Jul 2008 18:41:16 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"Frank Ch. Eigler" <fche@...hat.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: module-placed markers/tracepoints

* Frank Ch. Eigler (fche@...hat.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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