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Subject: Re: >Re: [RFC] should VM_BUG_ON(cond) really evaluate cond
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andi Kleen <andi@firstfloor.org>,
        Ben Hutchings <bhutchings@solarflare.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
Date: Sun, 30 Oct 2011 18:59:53 +0100
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Le dimanche 30 octobre 2011 à 10:48 -0700, Linus Torvalds a écrit :
> On Sun, Oct 30, 2011 at 10:41 AM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> >
> > Changing atomic_read(const atomic_t *v) prototype to
> > atomic_read(atomic_t *v) is not an option.
> 
> Why not?
> 
>     #define atomic_read(v)     ACCESS_AT_MOST_ONCE((v)->counter)
> 
> seems to be the cleanest thing.
> 

As I said, because v can be a const pointer provided by the caller.

Try it yourself and you'll discover hundred of call sites doing

.... some_function(const struct *xxx, ...)
{
	if (atomic_read(&xxx->refcnt) <= 0)
		do_something();
	else
		do_otherthing();
}

> And if you don't think this is "an option", I really can't see why you
> care about the extra instructions in the code stream either.
> 

Not an option if we have to change all callers that expected to be able
to use a const atomic_t pointer.

OK, I now have to leave the net.