Re: [PATCH v3 4/4] RDMA/cma: Avoid GID lookups on iWARP devices

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Jun 14, 2023 at 10:00:59AM -0400, Chuck Lever wrote:
> From: Chuck Lever <chuck.lever@oracle.com>
> 
> We would like to enable the use of siw on top of a VPN that is
> constructed and managed via a tun device. That hasn't worked up
> until now because ARPHRD_NONE devices (such as tun devices) have
> no GID for the RDMA/core to look up.
> 
> But it turns out that the egress device has already been picked for
> us -- no GID is necessary. addr_handler() just has to do the right
> thing with it.
> 
> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>  drivers/infiniband/core/cma.c |   13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/drivers/infiniband/core/cma.c b/drivers/infiniband/core/cma.c
> index a1756ed1faa1..50b8da2c4720 100644
> --- a/drivers/infiniband/core/cma.c
> +++ b/drivers/infiniband/core/cma.c
> @@ -700,6 +700,19 @@ cma_validate_port(struct ib_device *device, u32 port,
>  	if ((dev_type != ARPHRD_INFINIBAND) && rdma_protocol_ib(device, port))
>  		goto out;
>  
> +	if (rdma_protocol_iwarp(device, port)) {
> +		sgid_attr = rdma_get_gid_attr(device, port, 0);
> +		if (IS_ERR(sgid_attr))
> +			goto out;
> +
> +		/* XXX: I don't think this is RCU-safe, but does it need to be? */

Maybe for subtle reasons related to iwarp it is safe, but it should
make a lockdep splat since you are not holding rcu?

Remove the comment and do a rcu_lock/unlock around this and the deref

Jason