linux-kernel - Re: [PATCH] Module : call synchronize_sched() between module exit() and free.

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Message-ID: <20080730140938.GC18210@Krystal>
Date:	Wed, 30 Jul 2008 10:09:38 -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: [PATCH] Module : call synchronize_sched() between module
	exit() and free.

* Frank Ch. Eigler (fche@...hat.com) wrote:
> Hi -
> 
> On Tue, Jul 29, 2008 at 10:27:51PM -0400, Mathieu Desnoyers wrote:
> > [...]
> > 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.
> > [...]
> 
> Just to spell it out, it is this scenario I'd like to see documented:
> 
> module-foo.c:
>    foo() { ... trace_mark (foo, "..."); ... }
> 
> module-bar.c:
>    setup() { ... marker_probe_register ("foo" , ..., &foo_handler ); }
>    teardown() { ... marker_probe_unregister ("foo" , ..., &foo_handler ); }
>    foo_handler() { }
> 
> 1)  module-foo loads
> 2)  module-bar loads
> 3)  module-bar.c:setup()
> 4)  module-foo unloads
> 
> What happens here?  Certainly no more calls to foo_handler, but is
> that all?

Yep, that's it. However a hash table still keeps track of the "foo"
handler so that it's reconnected whenever module-foo is reloaded.

>(Would it not be desirable for an active marker to cause
> module-foo's refcount to increase, so as to prevent unloading at this
> time?)

No, because I want to be able to unload the marked module and I don't
want the fact that a probe is connected to it to change that.

> 
> 5)  module-bar.c:teardown()
> 
> Can this teardown code succeed fully even if module-foo is already
> dead and gone?
>   

Yes.

Actually there is a detail missing here. Your teardown should be :
 teardown() { ... marker_probe_unregister ("foo" , ..., &foo_handler );
                  synchronize_sched(); /* Before returning from exit */ }

This makes sure that every live marker call are finished and that it is
safe to unload module-bar (the probe).

Mathieu
> 
> - FChE

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