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

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Date:	Wed, 28 Jan 2009 16:05:03 -0800
From:	Roland Dreier <rdreier@...co.com>
To:	Andy Grover <andy.grover@...cle.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

 > +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.

 - R.
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