linux-kernel - Re: random.c changes for sparse irq

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Date:	Wed, 31 Dec 2008 17:40:26 -0600
From:	Matt Mackall <mpm@...enic.com>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...l.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: random.c changes for sparse irq_desc are crap

On Wed, 2008-12-31 at 15:07 -0800, Yinghai Lu wrote:
> Matt Mackall wrote:
> > I just noticed you merged a change that pointlessly converts two
> > random.c functions into ugly random.h inlines without going through the
> > maintainer.
> > 
> > I also don't like the look of the newly-introduced sparse variants of
> > these functions. Failure to find an irq descriptor in
> > get_timer_rand_state is a BUG_ON should-never-happen sort of condition,
> > not something to silently ignore. Letting the code try to dereference
> > NULL is preferred here: we'll actually be able to find and fix the
> > broken driver that's throwing around meaningless irq vectors.
> > 
> > Throwing away the timer_state pointer in the set_timer_rand_state
> > function is similarly bogus in addition to being a memory leak.
> > 
> > Please fix this up.
> > 
> 
> want something like this?

Not quite.

First, please turn these back into normal functions in random.c.
Inlining functions is generally discouraged these days unless you have a
good reason and numbers to back it up.

Second, as I tried to explain above, BUG_ON(!desc) is doing very little
that the subsequent desc->timer_rand_state wouldn't already do
(generating a traceback). In cases where dereferencing NULL should never
happen, it's preferable to not add the extra check and just let the oops
happen. We'll still get a backtrace if anything ever goes wrong, but we
won't have wasted any code space or CPU cycles when it doesn't.

-- 
Mathematics is the supreme nostalgia of our time.

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