linux-kernel - Re: [PATCH] refcount: always allow checked forms

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Date:   Wed, 4 Jul 2018 10:46:41 +0200
From:   David Sterba <dsterba@...e.cz>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org, Will Deacon <will.deacon@....com>,
        Kees Cook <keescook@...omium.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] refcount: always allow checked forms

On Tue, Jul 03, 2018 at 11:01:02AM +0100, Mark Rutland wrote:
> In many cases, it would be useful to be able to use the full
> sanity-checked refcount helpers regardless of CONFIG_REFCOUNT_FULL, as
> this would help to avoid duplicate warnings where callers try to
> sanity-check refcount manipulation.
> 
> This patch refactors things such that the full refcount helpers were
> always built, as refcount_${op}_checked(), such that they can be used
> regardless of CONFIG_REFCOUNT_FULL. This will allow code which *always*
> wants a checked refcount to opt-in, avoiding the need to duplicate the
> logic for warnings.
> 
> There should be no functional change as a result of this patch.
> 
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: David Sterba <dsterba@...e.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Will Deacon <will.deacon@....com>

I dare to give it my

Reviewed-by: David Sterba <dsterba@...e.com>

as my POC implementations were crap and Mark's version is much better.

> ---
>  include/linux/refcount.h | 27 +++++++++++++++++-------
>  lib/refcount.c           | 53 +++++++++++++++++++++++-------------------------
>  2 files changed, 45 insertions(+), 35 deletions(-)
> 
> Dave pointed out that it would be useful to be able to opt-in to full checks
> regardless of CONFIG_REFCOUNT_FULL, so that we can simplify callsites where we
> always want checks. I've spotted a few of these in code which is still awaiting
> conversion.

The motivation was code like

	WARN_ON(refcount_read(&ref));
	if (refcount_dec_and_test(&ref)) { ... }

so the warning is redundant for REFCOUNT_FULL, but I'm going to use the
_checked versions everywhere the performance of refcounts is not
critical.