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

["bfields@fieldses.org" <bfields@fieldses.org>]

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?

(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);
 	}