linux-kernel - Re: [PATCH 2/7] fs, xfs: convert xfs_buf.b_hold and xfs_buf.b_lru_ref from atomic_t to refcount

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Date:   Tue, 21 Feb 2017 17:04:08 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Elena Reshetova <elena.reshetova@...el.com>
Cc:     linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        gregkh@...uxfoundation.org, darrick.wong@...cle.com,
        Hans Liljestrand <ishkamiel@...il.com>,
        Kees Cook <keescook@...omium.org>,
        David Windsor <dwindsor@...il.com>
Subject: Re: [PATCH 2/7] fs, xfs: convert xfs_buf.b_hold and
 xfs_buf.b_lru_ref from atomic_t to refcount_t

On Tue, Feb 21, 2017 at 05:49:02PM +0200, Elena Reshetova wrote:
> @@ -1684,10 +1684,11 @@ xfs_buftarg_isolate(
>  	 * zero. If the value is already zero, we need to reclaim the
>  	 * buffer, otherwise it gets another trip through the LRU.
>  	 */
> -	if (!atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
> +	if (!refcount_read(&bp->b_lru_ref)) {
>  		spin_unlock(&bp->b_lock);
>  		return LRU_ROTATE;
>  	}
> +	refcount_dec_and_test(&bp->b_lru_ref);
>  
>  	bp->b_state |= XFS_BSTATE_DISPOSE;
>  	list_lru_isolate_move(lru, item, dispose);

This should never have passed testing.. refcount_dec_and_test() has a
__must_check.

Furthermore the above seems to suggest thingies can live with a 0
refcount, so a straight conversion cannot work.