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.

Chuck tried this patch and said "I don't see any performance
regressions, but my server has only a single last-level CPU cache."

Tested-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

3 files changed, 15 insertions, 3 deletions

diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
index a2435d3811a9..61530b1b7754 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();
         xpt_flags = 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..3ebb158c2279 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
@@ -314,8 +314,9 @@ static void svc_rdma_wc_receive(struct ib_cq *cq, struct ib_wc *wc)
 
         spin_lock(&rdma->sc_rq_dto_lock);
         list_add_tail(&ctxt->rc_list, &rdma->sc_rq_dto_q);
-        spin_unlock(&rdma->sc_rq_dto_lock);
+        /* Note the unlock pairs with the smp_rmb in svc_xprt_ready: */
         set_bit(XPT_DATA, &rdma->sc_xprt.xpt_flags);
+        spin_unlock(&rdma->sc_rq_dto_lock);
         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..c35753691960 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_rw.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_rw.c
@@ -287,9 +287,10 @@ static void svc_rdma_wc_read_done(struct ib_cq *cq, struct ib_wc *wc)
                 spin_lock(&rdma->sc_rq_dto_lock);
                 list_add_tail(&info->ri_readctxt->rc_list,
                               &rdma->sc_read_complete_q);
+                /* Note the unlock pairs with the smp_rmb in svc_xprt_ready: */
+                set_bit(XPT_DATA, &rdma->sc_xprt.xpt_flags);
                 spin_unlock(&rdma->sc_rq_dto_lock);
 
-                set_bit(XPT_DATA, &rdma->sc_xprt.xpt_flags);
                 svc_xprt_enqueue(&rdma->sc_xprt);
         }