netdev - Re: [Patch net-next v2] mlx5: use RCU lock in mlx5_eq_cq

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Message-ID: <e0a0e868-09f6-209f-31a2-83a2eb4aba24@gmail.com>
Date:   Wed, 6 Feb 2019 15:55:10 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Cong Wang <xiyou.wangcong@...il.com>, netdev@...r.kernel.org
Cc:     Saeed Mahameed <saeedm@...lanox.com>,
        Tariq Toukan <tariqt@...lanox.com>
Subject: Re: [Patch net-next v2] mlx5: use RCU lock in mlx5_eq_cq_get()



On 02/06/2019 03:36 PM, Eric Dumazet wrote:
> 

> I suspect that you need a variant that makes sure refcount is not zero.
> 
> ( Typical RCU rules apply )
> 
> if (cq && !refcount_inc_not_zero(&cq->refcount))
> 	cq = NULL;
> 
> 
> See commit 6fa19f5637a6c22bc0999596bcc83bdcac8a4fa6 rds: fix refcount bug in rds_sock_addref
> for a similar issue I fixed recently.
> 
 
By the way, we also could avoid two atomics on the cq->refcount , by using rcu_read_lock()/unlock()
in the two callers .