linux-kernel - Re: [PATCH] Detect and warn on atomic_inc/atomic

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Message-ID: <20090430082350.GA21699@elte.hu>
Date:	Thu, 30 Apr 2009 10:23:50 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Nikanth Karthikesan <knikanth@...ell.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Detect and warn on atomic_inc/atomic_dec wrapping
	around

* Nikanth Karthikesan <knikanth@...ell.com> wrote:

> Add a debug option to detect and warn when the 32-bit atomic_t 
> wraps around during atomic_inc and atomic_dec.
> 
> Signed-off-by: Nikanth Karthikesan <knikanth@...e.de>

hm, what's the motivation?

As a generic debug helper this is not appropriate i think - counts 
can easily have a meaning when going negative as well. (we have no 
signed-atomic primitives)

>  static inline void atomic_inc(atomic_t *v)
>  {
> +#if defined(CONFIG_ENABLE_WARN_ATOMIC_INC_WRAP)
> +	WARN_ON(atomic_add_unless(v, 1, INT_MAX) == 0);
> +#else
>  	asm volatile(LOCK_PREFIX "incl %0"
>  		     : "+m" (v->counter));
> +#endif
>  }

also looks a bit ugly - this ugly #ifdef would spread into every 
architecture.

If we want to restrict atomic_t value ranges like that then the 
clean solution would be to add generic wrappers doing the debug 
(once, in generic code), and renaming the arch primitives to 
raw_atomic_inc() (etc), doing the lowlevel bits cleanly.

Do we really want this?

	Ingo
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