linux-kernel - Re: [Cocci] [PATCH 1/1] kprobes: Deletion of an unnecessary check before the function call "module

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Message-ID: <alpine.DEB.2.10.1411161639310.2364@hadrien>
Date:	Sun, 16 Nov 2014 16:43:51 +0100 (CET)
From:	Julia Lawall <julia.lawall@...6.fr>
To:	SF Markus Elfring <elfring@...rs.sourceforge.net>
cc:	Julia Lawall <julia.lawall@...6.fr>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
	"David S. Miller" <davem@...emloft.net>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org,
	Coccinelle <cocci@...teme.lip6.fr>
Subject: Re: [Cocci] [PATCH 1/1] kprobes: Deletion of an unnecessary check
 before the function call "module_put"

On Sun, 16 Nov 2014, SF Markus Elfring wrote:

> >> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> >> index 3995f54..f1e7d45 100644
> >> --- a/kernel/kprobes.c
> >> +++ b/kernel/kprobes.c
> >> @@ -1527,8 +1527,7 @@ int register_kprobe(struct kprobe *p)
> >>  out:
> >>  	mutex_unlock(&kprobe_mutex);
> >>
> >> -	if (probed_mod)
> >> -		module_put(probed_mod);
> >> +	module_put(probed_mod);
> >
> > There is an out label, so please check whether the labels could not be
> > better positioned to avoid calling module_put when it is not needed.
>
> I do not see refactoring opportunities around jump labels in this use case
> for the implementation of the register_kprobe() function so far because
> the mutex_unlock() function must be called.
> Would you like to suggest any other source code fine-tuning?

OK.  I don't think that removing the if is a good choice in this case.
The code ret = check_kprobe_address_safe(p, &probed_mod); is unusual, in
that it can fail to do anything in two ways.  One is by setting ret, on
detecting an error, and the other is by returning 0 but still putting a
NULL value in probed_mod when there is nothing to do.  Thus, in the
successful execution of the rest of the function, a probed module might or
might not exist.  The if around the module_put is helpful to the reader to
understand that this possibility exists.

julia
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