netdev - Re: [PATCH 17/21] RDS/IB: Receive datagrams via IB

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Date:	Wed, 28 Jan 2009 18:20:40 -0800
From:	Andy Grover <andy.grover@...cle.com>
To:	Roland Dreier <rdreier@...co.com>
CC:	rds-devel@....oracle.com, general@...ts.openfabrics.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH 17/21] RDS/IB: Receive datagrams via IB

Roland Dreier wrote:
>  > +static int rds_ib_recv_refill_one(struct rds_connection *conn,
>  > +				  struct rds_ib_recv_work *recv,
>  > +				  gfp_t kptr_gfp, gfp_t page_gfp)
>  > +{
>  > +	struct rds_ib_connection *ic = conn->c_transport_data;
>  > +	dma_addr_t dma_addr;
>  > +	struct ib_sge *sge;
>  > +	int ret = -ENOMEM;
>  > +
>  > +	if (recv->r_ibinc == NULL) {
>  > +		if (atomic_read(&rds_ib_allocation) >= rds_ib_sysctl_max_recv_allocation) {
>  > +			rds_ib_stats_inc(s_ib_rx_alloc_limit);
>  > +			goto out;
>  > +		}
>  > +		recv->r_ibinc = kmem_cache_alloc(rds_ib_incoming_slab,
>  > +						 kptr_gfp);
>  > +		if (recv->r_ibinc == NULL)
>  > +			goto out;
>  > +		atomic_inc(&rds_ib_allocation);
> 
> This is racy.  You check if you're at the limit, do the allocation, and
> then increment the atomic rds_ib_allocation count.  So many threads can
> pass the atomic_read() test and then take you over the limit.  If you
> want to make it safe then you could do atomic_inc_return() and check if
> that took you over the limit.

Woah, yup, thanks.

-- Andy
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