linux-kernel - Re: [PATCH 12/19] kernel: convert css_set.refcount from atomic_t to refcount

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Message-ID: <20170306195439.GD19696@htj.duckdns.org>
Date:   Mon, 6 Mar 2017 14:54:39 -0500
From:   Tejun Heo <tj@...nel.org>
To:     Elena Reshetova <elena.reshetova@...el.com>
Cc:     linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-audit@...hat.com, linux-fsdevel@...r.kernel.org,
        peterz@...radead.org, gregkh@...uxfoundation.org,
        viro@...iv.linux.org.uk, mingo@...hat.com, hannes@...xchg.org,
        lizefan@...wei.com, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, paul@...l-moore.com,
        eparis@...hat.com, akpm@...ux-foundation.org, arnd@...db.de,
        luto@...nel.org, Hans Liljestrand <ishkamiel@...il.com>,
        Kees Cook <keescook@...omium.org>,
        David Windsor <dwindsor@...il.com>
Subject: Re: [PATCH 12/19] kernel: convert css_set.refcount from atomic_t to
 refcount_t

Hello,

On Mon, Feb 20, 2017 at 12:19:01PM +0200, Elena Reshetova wrote:
> @@ -134,10 +135,13 @@ static inline void put_css_set(struct css_set *cset)
>  	 * can see it. Similar to atomic_dec_and_lock(), but for an
>  	 * rwlock
>  	 */
> -	if (atomic_add_unless(&cset->refcount, -1, 1))
> +	spin_lock_irqsave(&css_set_lock, flags);
> +	if (refcount_read(&cset->refcount) != 1) {
> +		WARN_ON(refcount_dec_and_test(&cset->refcount));
> +		spin_unlock_irqrestore(&css_set_lock, flags);
>  		return;
> +	}

This isn't an equivalent conversion and should have been mentioned in
the patch description.  Hmm... and I'm not sure this is a good idea.
Can't we add the matching operation on refcount_t rather than adding
extra locking like this?

Thanks.

-- 
tejun