linux-kernel - Re: [PATCH for-next v6 7/7] RDMA/rxe: Add support for the traditional Atomic operations with ODP

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Date:   Fri, 8 Sep 2023 11:29:01 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Daisuke Matsuda <matsuda-daisuke@...itsu.com>
Cc:     linux-rdma@...r.kernel.org, leon@...nel.org, zyjzyj2000@...il.com,
        linux-kernel@...r.kernel.org, rpearsonhpe@...il.com,
        yangx.jy@...itsu.com, lizhijian@...itsu.com, y-goto@...itsu.com
Subject: Re: [PATCH for-next v6 7/7] RDMA/rxe: Add support for the
 traditional Atomic operations with ODP

On Fri, Sep 08, 2023 at 03:26:48PM +0900, Daisuke Matsuda wrote:
> +int rxe_odp_mr_atomic_op(struct rxe_mr *mr, u64 iova, int opcode,
> +			 u64 compare, u64 swap_add, u64 *orig_val)
> +{
> +	int err;
> +	int retry = 0;
> +	struct ib_umem_odp *umem_odp = to_ib_umem_odp(mr->umem);
> +
> +	mutex_lock(&umem_odp->umem_mutex);
> +
> +	/* Atomic operations manipulate a single char. */
> +	if (rxe_odp_check_pages(mr, iova, sizeof(char), 0))
> +		goto need_fault;
> +
> +	err = rxe_mr_do_atomic_op(mr, iova, opcode, compare,
> +				  swap_add, orig_val);
> +
> +	mutex_unlock(&umem_odp->umem_mutex);

You should just use the xarray spinlock, the umem_mutex should only be
held around the faulting flow

> +
> +	return err;
> +
> +need_fault:
> +	/* allow max 3 tries for pagefault */
> +	do {

Why a retry loop? We already have a retry loop in
ib_umem_odp_map_dma_and_lock,it doesn't need to be done externally. If
you reach here with the lock held then progress should be guarenteed
under the lock.

Jason