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Subject: Re: [PATCH] SUNRPC: Don't allow compiler optimisation of
 svc_xprt_release_slot()
From:   Chuck Lever <chuck.lever@oracle.com>
In-Reply-To: <20190111211235.GA27206@fieldses.org>
Date:   Fri, 11 Jan 2019 16:52:21 -0500
Cc:     Trond Myklebust <trondmy@hammerspace.com>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>
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References: <20190103141712.24381-1-trond.myklebust@hammerspace.com>
 <20190103224529.GA6907@fieldses.org>
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 <20190108150107.GA15921@fieldses.org>
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 <20190111211235.GA27206@fieldses.org>
To:     Bruce Fields <bfields@fieldses.org>
Sender: linux-nfs-owner@vger.kernel.org
Precedence: bulk


> On Jan 11, 2019, at 4:12 PM, bfields@fieldses.org wrote:
>=20
> On Wed, Jan 09, 2019 at 05:41:36PM +0000, Trond Myklebust wrote:
>> The above is stating that
>>=20
>> smp_rmb();
>> smp_read_barrier_depends();
>> if (xprt->xpt_flags & ....)
>>=20
>> is redundant and can be replaced with just
>>=20
>> smp_rmb();
>> if (xprt->xpt_flags & ....)
>>=20
>> However that's not the case for smp_rmb() followed by READ_ONCE(). =
That
>> would expand to
>>=20
>> smp_rmb();
>> if (xprt->xpt_flags & ...) {
>>    smp_read_barrier_depends();
>> } else
>>    smp_read_barrier_depends();
>>=20
>> 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.
>>=20
>> See, for instance, kernel/events/core.c which has several examples, =
or
>> kernel/exit.c.
>=20
> You're right, I was confused.
>=20
> So, I think we need your patch plus something like this.
>=20
> 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().)
>=20
> --b.
>=20
> commit d7356c3250d4
> Author: J. Bruce Fields <bfields@redhat.com>
> Date:   Fri Jan 11 15:36:40 2019 -0500
>=20
>    svcrpc: fix unlikely races preventing queueing of sockets
>=20
>    In the rpc server, When something happens that might be reason to =
wake
>    up a thread to do something, what we do is
>=20
>            - 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.
>=20
>    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.
>=20
>    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.
>=20
>    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.
>=20
>    Signed-off-by: J. Bruce Fields <bfields@redhat.com>
>=20
> 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 =3D 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;
>=20
> +	/*
> +	 * 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);
>=20
> 	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 =3D 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);
>=20
> 		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);
> 	}
>=20

--
Chuck Lever