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

Chuck Lever
2019-01-11T21:52:32Z

> On Jan 11, 2019, at 4:12 PM, bfields@fieldses.org wrote:
>
> On Wed, Jan 09, 2019 at 05:41:36PM +0000, Trond Myklebust wrote:
>> The above is stating that
>>
>> smp_rmb();
>> smp_read_barrier_depends();
>> if (xprt->xpt_flags & ....)
>>
>> is redundant and can be replaced with just
>>
>> smp_rmb();
>> if (xprt->xpt_flags & ....)
>>
>> However that's not the case for smp_rmb() followed by READ_ONCE(). That
>> would expand to
>>
>> smp_rmb();
>> if (xprt->xpt_flags & ...) {
>> smp_read_barrier_depends();
>> } else
>> smp_read_barrier_depends();
>>
>> which is not redundant. It is ensuring (on alpha only) that the read of
>> xprt->xpt_flags is also not re-ordered w.r.t. other data reads that
>> follow.
>>
>> See, for instance, kernel/events/core.c which has several examples, or
>> kernel/exit.c.
>
> You're right, I was confused.
>
> 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?
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?

> (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