linux-kernel - Re: [PATCH v2 2/2] cxgb4: smt: Use refcount

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Date:   Mon, 05 Aug 2019 10:55:00 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     hslester96@...il.com
Cc:     vishal@...lsio.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] cxgb4: smt: Use refcount_t for refcount

From: Chuhong Yuan <hslester96@...il.com>
Date: Fri,  2 Aug 2019 16:35:47 +0800

> refcount_t is better for reference counters since its
> implementation can prevent overflows.
> So convert atomic_t ref counters to refcount_t.
> 
> Signed-off-by: Chuhong Yuan <hslester96@...il.com>
> ---
> Changes in v2:
>   - Convert refcount from 0-base to 1-base.

The existing code is buggy and should be fixed before you start making
conversions to it.

> @@ -111,7 +111,7 @@ static void t4_smte_free(struct smt_entry *e)
>   */
>  void cxgb4_smt_release(struct smt_entry *e)
>  {
> -	if (atomic_dec_and_test(&e->refcnt))
> +	if (refcount_dec_and_test(&e->refcnt))
>  		t4_smte_free(e);

This runs without any locking and therefore:

>  	if (e) {
>  		spin_lock(&e->lock);
> -		if (!atomic_read(&e->refcnt)) {
> -			atomic_set(&e->refcnt, 1);
> +		if (refcount_read(&e->refcnt) == 1) {
> +			refcount_set(&e->refcnt, 2);

This test is not safe, since the reference count can asynchronously decrement
to zero above outside of any locks.

Then you'll need to add locking, and as a result the need to an atomic
counter goes away and just a normal int can be used.