svcrpc: document lack of some memory barriers

We're missing memory barriers in net/sunrpc/svcsock.c in some spots we'd
expect them.  But it doesn't appear they're necessary in our case, and
this is likely a hot path--for now just document the odd behavior.

Kosuke Tatsukawa found this issue while looking through the linux source
code for places calling waitqueue_active() before wake_up*(), but
without preceding memory barriers, after sending a patch to fix a
similar issue in drivers/tty/n_tty.c  (Details about the original issue
can be found here: https://lkml.org/lkml/2015/9/28/849).

Reported-by: Kosuke Tatsukawa <tatsu@ab.jp.nec.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

1 files changed, 31 insertions, 6 deletions

diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
index e0c7b3355495..1413cdcc131c 100644
--- a/net/sunrpc/svcsock.c
+++ b/net/sunrpc/svcsock.c
@@ -399,6 +399,31 @@ static int svc_sock_secure_port(struct svc_rqst *rqstp)
         return svc_port_is_privileged(svc_addr(rqstp));
 }
 
+static bool sunrpc_waitqueue_active(wait_queue_head_t *wq)
+{
+        if (!wq)
+                return false;
+        /*
+         * There should normally be a memory * barrier here--see
+         * wq_has_sleeper().
+         *
+         * It appears that isn't currently necessary, though, basically
+         * because callers all appear to have sufficient memory barriers
+         * between the time the relevant change is made and the
+         * time they call these callbacks.
+         *
+         * The nfsd code itself doesn't actually explicitly wait on
+         * these waitqueues, but it may wait on them for example in
+         * sendpage() or sendmsg() calls.  (And those may be the only
+         * places, since it it uses nonblocking reads.)
+         *
+         * Maybe we should add the memory barriers anyway, but these are
+         * hot paths so we'd need to be convinced there's no sigificant
+         * penalty.
+         */
+        return waitqueue_active(wq);
+}
+
 /*
  * INET callback when data has been received on the socket.
  */
@@ -414,7 +439,7 @@ static void svc_udp_data_ready(struct sock *sk)
                 set_bit(XPT_DATA, &svsk->sk_xprt.xpt_flags);
                 svc_xprt_enqueue(&svsk->sk_xprt);
         }
-        if (wq && waitqueue_active(wq))
+        if (sunrpc_waitqueue_active(wq))
                 wake_up_interruptible(wq);
 }
 
@@ -432,7 +457,7 @@ static void svc_write_space(struct sock *sk)
                 svc_xprt_enqueue(&svsk->sk_xprt);
         }
 
-        if (wq && waitqueue_active(wq)) {
+        if (sunrpc_waitqueue_active(wq)) {
                 dprintk("RPC svc_write_space: someone sleeping on %p\n",
                        svsk);
                 wake_up_interruptible(wq);
@@ -787,7 +812,7 @@ static void svc_tcp_listen_data_ready(struct sock *sk)
         }
 
         wq = sk_sleep(sk);
-        if (wq && waitqueue_active(wq))
+        if (sunrpc_waitqueue_active(wq))
                 wake_up_interruptible_all(wq);
 }
 
@@ -808,7 +833,7 @@ static void svc_tcp_state_change(struct sock *sk)
                 set_bit(XPT_CLOSE, &svsk->sk_xprt.xpt_flags);
                 svc_xprt_enqueue(&svsk->sk_xprt);
         }
-        if (wq && waitqueue_active(wq))
+        if (sunrpc_waitqueue_active(wq))
                 wake_up_interruptible_all(wq);
 }
 
@@ -823,7 +848,7 @@ static void svc_tcp_data_ready(struct sock *sk)
                 set_bit(XPT_DATA, &svsk->sk_xprt.xpt_flags);
                 svc_xprt_enqueue(&svsk->sk_xprt);
         }
-        if (wq && waitqueue_active(wq))
+        if (sunrpc_waitqueue_active(wq))
                 wake_up_interruptible(wq);
 }
 
@@ -1593,7 +1618,7 @@ static void svc_sock_detach(struct svc_xprt *xprt)
         sk->sk_write_space = svsk->sk_owspace;
 
         wq = sk_sleep(sk);
-        if (wq && waitqueue_active(wq))
+        if (sunrpc_waitqueue_active(wq))
                 wake_up_interruptible(wq);
 }