Re: [PATCH] SUNRPC: Don't allow compiler optimisation of svc_xprt_release_slot()

Chuck Lever
2019-01-11T22:27:40Z

> On Jan 11, 2019, at 5:10 PM, Bruce Fields <bfields@fieldses.org> wrote:
>
> On Fri, Jan 11, 2019 at 04:54:01PM -0500, Chuck Lever wrote:
>>> On Jan 11, 2019, at 4:52 PM, Chuck Lever <chuck.lever@oracle.com> wrote:
>>>> So, I think we need your patch plus something like this.
>>>>
>>>> Chuck, maybe you could help me with the "XXX: Chuck:" parts?
>>>
>>> I haven't been following. Why do you think those are necessary?
>
> I'm worried something like this could happen:
>
> CPU 1 CPU 2
> ----- -----
>
> set XPT_DATA dec xpt_nr_rqsts
>
> svc_xprt_enqueue svc_xprt_enqueue
>
> And both decide nothing should be done if neither sees the change that
> the other made.
>
> Maybe I'm still missing some reason that couldn't happen.
>
> Even if it can happen, it's an unlikely race that will likely be fixed
> when another event comes along a little later, which would explain why
> we've never seen any reports.
>
>>> We've had set_bit and atomic_{inc,dec} in this code for ages,
>>> and I've never noticed a problem.
>>>
>>> Rather than adding another CPU pipeline bubble in the RDMA code,
>>> though, could you simply move the set_bit() call site inside the
>>> critical sections?
>>
>> er, inside the preceding critical section. Just reverse the order
>> of the spin_unlock and the set_bit.
>
> That'd do it, thanks!

I can try that here and see if it results in a performance regression.

> --b.
>
>>
>>
>>>
>>>
>>>> (This applies on top of your patch plus another that just renames the
>>>> stupidly long svc_xprt_has_something_to_do() to svc_xprt_read().)
>>>>
>>>> --b.
>>>>
>>>> commit d7356c3250d4
>>>> Author: J. Bruce Fields <bfields@redhat.com>
>>>> Date: Fri Jan 11 15:36:40 2019 -0500
>>>>
>>>> svcrpc: fix unlikely races preventing queueing of sockets
>>>>
>>>> In the rpc server, When something happens that might be reason to wake
>>>> up a thread to do something, what we do is
>>>>
>>>> - modify xpt_flags, sk_sock->flags, xpt_reserved, or
>>>> xpt_nr_rqsts to indicate the new situation
>>>> - call svc_xprt_enqueue() to decide whether to wake up a thread.
>>>>
>>>> svc_xprt_enqueue may require multiple conditions to be true before
>>>> queueing up a thread to handle the xprt. In the SMP case, one of the
>>>> other CPU's may have set another required condition, and in that case,
>>>> although both CPUs run svc_xprt_enqueue(), it's possible that neither
>>>> call sees the writes done by the other CPU in time, and neither one
>>>> recognizes that all the required conditions have been set. A socket
>>>> could therefore be ignored indefinitely.
>>>>
>>>> Add memory barries to ensure that any svc_xprt_enqueue() call will
>>>> always see the conditions changed by other CPUs before deciding to
>>>> ignore a socket.
>>>>
>>>> I've never seen this race reported. In the unlikely event it happens,
>>>> another event will usually come along and the problem will fix itself.
>>>> So I don't think this is worth backporting to stable.
>>>>
>>>> Signed-off-by: J. Bruce Fields <bfields@redhat.com>
>>>>
>>>> diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
>>>> index d410ae512b02..2af21b84b3b6 100644
>>>> --- a/net/sunrpc/svc_xprt.c
>>>> +++ b/net/sunrpc/svc_xprt.c
>>>> @@ -357,6 +357,7 @@ static void svc_xprt_release_slot(struct svc_rqst *rqstp)
>>>> struct svc_xprt *xprt = rqstp->rq_xprt;
>>>> if (test_and_clear_bit(RQ_DATA, &rqstp->rq_flags)) {
>>>> atomic_dec(&xprt->xpt_nr_rqsts);
>>>> + smp_wmb(); /* See smp_rmb() in svc_xprt_ready() */
>>>> svc_xprt_enqueue(xprt);
>>>> }
>>>> }
>>>> @@ -365,6 +366,15 @@ static bool svc_xprt_ready(struct svc_xprt *xprt)
>>>> {
>>>> unsigned long xpt_flags;
>>>>
>>>> + /*
>>>> + * If another cpu has recently updated xpt_flags,
>>>> + * sk_sock->flags, xpt_reserved, or xpt_nr_rqsts, we need to
>>>> + * know about it; otherwise it's possible that both that cpu and
>>>> + * this one could call svc_xprt_enqueue() without either
>>>> + * svc_xprt_enqueue() recognizing that the conditions below
>>>> + * are satisfied, and we could stall indefinitely:
>>>> + */
>>>> + smp_rmb();
>>>> READ_ONCE(xprt->xpt_flags);
>>>>
>>>> if (xpt_flags & (BIT(XPT_CONN) | BIT(XPT_CLOSE)))
>>>> @@ -479,7 +489,7 @@ void svc_reserve(struct svc_rqst *rqstp, int space)
>>>> if (xprt && space < rqstp->rq_reserved) {
>>>> atomic_sub((rqstp->rq_reserved - space), &xprt->xpt_reserved);
>>>> rqstp->rq_reserved = space;
>>>> -
>>>> + smp_wmb(); /* See smp_rmb() in svc_xprt_ready() */
>>>> svc_xprt_enqueue(xprt);
>>>> }
>>>> }
>>>> diff --git a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
>>>> index 828b149eaaef..377244992ae8 100644
>>>> --- a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
>>>> +++ b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
>>>> @@ -316,6 +316,7 @@ static void svc_rdma_wc_receive(struct ib_cq *cq, struct ib_wc *wc)
>>>> list_add_tail(&ctxt->rc_list, &rdma->sc_rq_dto_q);
>>>> spin_unlock(&rdma->sc_rq_dto_lock);
>>>> set_bit(XPT_DATA, &rdma->sc_xprt.xpt_flags);
>>>> + /* XXX: Chuck: do we need an smp_mb__after_atomic() here? */
>>>> if (!test_bit(RDMAXPRT_CONN_PENDING, &rdma->sc_flags))
>>>> svc_xprt_enqueue(&rdma->sc_xprt);
>>>> goto out;
>>>> diff --git a/net/sunrpc/xprtrdma/svc_rdma_rw.c b/net/sunrpc/xprtrdma/svc_rdma_rw.c
>>>> index dc1951759a8e..e1a790487d69 100644
>>>> --- a/net/sunrpc/xprtrdma/svc_rdma_rw.c
>>>> +++ b/net/sunrpc/xprtrdma/svc_rdma_rw.c
>>>> @@ -290,6 +290,7 @@ static void svc_rdma_wc_read_done(struct ib_cq *cq, struct ib_wc *wc)
>>>> spin_unlock(&rdma->sc_rq_dto_lock);
>>>>
>>>> set_bit(XPT_DATA, &rdma->sc_xprt.xpt_flags);
>>>> + /* XXX: Chuck: do we need a smp_mb__after_atomic() here? */
>>>> svc_xprt_enqueue(&rdma->sc_xprt);
>>>> }
>>>>
>>>
>>> --
>>> Chuck Lever
>>
>> --
>> Chuck Lever
>>
>>

--
Chuck Lever