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

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

On Mon, Jul 27, 2009 at 04:43:21PM -0500, Tom Tucker wrote:
> Please disregard this mail! I fixed a relayhost parameter in a lab  
> machine and this queued mail was sent. My apologies.

No problem!  I didn't recognize that first patch, but then got to the
following series and said hey, wait a minute, we already did this....

--b.

>
> Tom
>
> J. Bruce Fields wrote:
>> On Mon, Jul 27, 2009 at 05:33:55PM -0400, bfields wrote:
>>   
>>> 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.
>>>     
>>
>> Erp, cripes, sorry--only just noticed all this mail seems to have been
>> delayed a year!  OK....--b.
>>
>>   
>>> --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;
>>>>       
>> --
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