Re: [PATCH] svcrdma: Fix race between svc_rdma_recvfrom thread and the dto_tasklet

["J. Bruce Fields" <bfields@fieldses.org>]

On Tue, Aug 12, 2008 at 05:13:04PM -0500, Tom Tucker wrote:
> RDMA_READ completions are kept on a separate queue from the general
> I/O request queue. Since a separate lock is used to protect the RDMA_READ
> completion queue, a race exists between the dto_tasklet and the
> svc_rdma_recvfrom thread where the dto_tasklet sets the XPT_DATA
> bit and adds I/O to the read-completion queue. Concurrently, the
> recvfrom thread checks the generic queue, finds it empty and resets
> the XPT_DATA bit. A subsequent svc_xprt_enqueue will fail to enqueue
> the transport for I/O and cause the transport to "stall".
> 
> The fix is to protect both lists with the same lock and set the XPT_DATA
> bit with this lock held.

Thanks.  Assuming this race has existed from the start (so it's not a
recent regression), and the consequences are no worse than a stall, I'm
inclined to queue this up for 2.6.32--but if I'm wrong about either of
those, we could try for 2.6.31 and/or stable.

--b.

> 
> Signed-off-by: Tom Tucker <tom@opengridcomputing.com>
> 
> ---
>  include/linux/sunrpc/svc_rdma.h          |    1 -
>  net/sunrpc/xprtrdma/svc_rdma_recvfrom.c  |    8 ++++----
>  net/sunrpc/xprtrdma/svc_rdma_transport.c |    5 ++---
>  3 files changed, 6 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/sunrpc/svc_rdma.h b/include/linux/sunrpc/svc_rdma.h
> index ef2e3a2..dc05b54 100644
> --- a/include/linux/sunrpc/svc_rdma.h
> +++ b/include/linux/sunrpc/svc_rdma.h
> @@ -143,7 +143,6 @@ struct svcxprt_rdma {
>  	unsigned long	     sc_flags;
>  	struct list_head     sc_dto_q;		/* DTO tasklet I/O pending Q */
>  	struct list_head     sc_read_complete_q;
> -	spinlock_t           sc_read_complete_lock;
>  	struct work_struct   sc_work;
>  };
>  /* sc_flags */
> diff --git a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
> index b4b17f4..74de31a 100644
> --- a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
> +++ b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
> @@ -443,18 +443,18 @@ int svc_rdma_recvfrom(struct svc_rqst *rqstp)
>  
>  	dprintk("svcrdma: rqstp=%p\n", rqstp);
>  
> -	spin_lock_bh(&rdma_xprt->sc_read_complete_lock);
> +	spin_lock_bh(&rdma_xprt->sc_rq_dto_lock);
>  	if (!list_empty(&rdma_xprt->sc_read_complete_q)) {
>  		ctxt = list_entry(rdma_xprt->sc_read_complete_q.next,
>  				  struct svc_rdma_op_ctxt,
>  				  dto_q);
>  		list_del_init(&ctxt->dto_q);
>  	}
> -	spin_unlock_bh(&rdma_xprt->sc_read_complete_lock);
> -	if (ctxt)
> +	if (ctxt) {
> +		spin_unlock_bh(&rdma_xprt->sc_rq_dto_lock);
>  		return rdma_read_complete(rqstp, ctxt);
> +	}
>  
> -	spin_lock_bh(&rdma_xprt->sc_rq_dto_lock);
>  	if (!list_empty(&rdma_xprt->sc_rq_dto_q)) {
>  		ctxt = list_entry(rdma_xprt->sc_rq_dto_q.next,
>  				  struct svc_rdma_op_ctxt,
> diff --git a/net/sunrpc/xprtrdma/svc_rdma_transport.c b/net/sunrpc/xprtrdma/svc_rdma_transport.c
> index 19ddc38..900cb69 100644
> --- a/net/sunrpc/xprtrdma/svc_rdma_transport.c
> +++ b/net/sunrpc/xprtrdma/svc_rdma_transport.c
> @@ -359,11 +359,11 @@ static void sq_cq_reap(struct svcxprt_rdma *xprt)
>  			if (test_bit(RDMACTXT_F_LAST_CTXT, &ctxt->flags)) {
>  				struct svc_rdma_op_ctxt *read_hdr = ctxt->read_hdr;
>  				BUG_ON(!read_hdr);
> +				spin_lock_bh(&xprt->sc_rq_dto_lock);
>  				set_bit(XPT_DATA, &xprt->sc_xprt.xpt_flags);
> -				spin_lock_bh(&xprt->sc_read_complete_lock);
>  				list_add_tail(&read_hdr->dto_q,
>  					      &xprt->sc_read_complete_q);
> -				spin_unlock_bh(&xprt->sc_read_complete_lock);
> +				spin_unlock_bh(&xprt->sc_rq_dto_lock);
>  				svc_xprt_enqueue(&xprt->sc_xprt);
>  			}
>  			svc_rdma_put_context(ctxt, 0);
> @@ -428,7 +428,6 @@ static struct svcxprt_rdma *rdma_create_xprt(struct svc_serv *serv,
>  	init_waitqueue_head(&cma_xprt->sc_send_wait);
>  
>  	spin_lock_init(&cma_xprt->sc_lock);
> -	spin_lock_init(&cma_xprt->sc_read_complete_lock);
>  	spin_lock_init(&cma_xprt->sc_rq_dto_lock);
>  
>  	cma_xprt->sc_ord = svcrdma_ord;