Re: [PATCH v2] sunrpc: fix waitqueue_active without memory barrier in sunrpc

Tatsukawa Kosuke wrote:

J. Bruce Fields wrote:

quoted

On Thu, Oct 15, 2015 at 11:44:20AM +0000, Kosuke Tatsukawa wrote:

quoted

Tatsukawa Kosuke wrote:

quoted

J. Bruce Fields wrote:

quoted

Thanks for the detailed investigation.

I think it would be worth adding a comment if that might help someone
having to reinvestigate this again some day.

It would be nice, but I find it difficult to write a comment in the
sunrpc layer why a memory barrier isn't necessary, using the knowledge
of how nfsd uses it, and the current implementation of the network code.

Personally, I would prefer removing the call to waitqueue_active() which
would make the memory barrier totally unnecessary at the cost of a
spin_lock + spin_unlock by unconditionally calling
wake_up_interruptible.

On second thought, the callbacks will be called frequently from the tcp
code, so it wouldn't be a good idea.

So, I was even considering documenting it like this, if it's not
overkill.

Hmm... but if this is right, then we may as well ask why we're doing the
wakeups at all.  Might be educational to test the code with them
removed.

sk_write_space will be called in sock_wfree() with UDP/IP each time
kfree_skb() is called.  With TCP/IP, sk_write_space is only called if
SOCK_NOSPACE has been set.

sk_data_ready will be called in both tcp_rcv_established() for TCP/IP
and in sock_queue_rcv_skb() for UDP/IP.  The latter lacks a memory
barrier with sk_data_ready called right after __skb_queue_tail().
I think this hasn't caused any problems because sk_data_ready wasn't
used.

Actually, svc_udp_data_ready() calls set_bit() which is an atomic
operation.  So there won't be a problem unless svsk is NULL.

quoted

--b.

commit 0882cfeb39e0
Author: J. Bruce Fields [off-list ref]
Date:   Thu Oct 15 16:53:41 2015 -0400

    svcrpc: document lack of some memory barriers.
    
    Kosuke Tatsukawa points out an odd lack of memory barriers in some sites
    here.  I think the code's correct, but it's probably worth documenting.
    
    Reported-by: Kosuke Tatsukawa [off-list ref]

diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
index 856407fa085e..90480993ec4a 100644
--- a/net/sunrpc/svcsock.c
+++ b/net/sunrpc/svcsock.c

@@ -399,6 +399,25 @@ static int svc_sock_secure_port(struct svc_rqst *rqstp)
 	return svc_port_is_privileged(svc_addr(rqstp));
 }
 
+static void svc_no_smp_mb(void)
+{
+	/*
+	 * Kosuke Tatsukawa points out there should normally be an
+	 * smp_mb() at the callsites of this function.  (Either that or
+	 * we could just drop the waitqueue_active() checks.)
+	 *
+	 * It appears they aren't currently necessary, though, basically
+	 * because nfsd does non-blocking reads from these sockets, so
+	 * the only places we wait on this waitqueue is in sendpage and
+	 * sendmsg, which won't be waiting for wakeups on newly arrived
+	 * data.
+	 *
+	 * 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.
+	 */
+}
+
 /*
  * INET callback when data has been received on the socket.
  */

@@ -414,7 +433,7 @@ static void svc_udp_data_ready(struct sock *sk)
 		set_bit(XPT_DATA, &svsk->sk_xprt.xpt_flags);
 		svc_xprt_enqueue(&svsk->sk_xprt);
 	}
-	smp_mb();
+	svc_no_smp_mb();
 	if (wq && waitqueue_active(wq))
 		wake_up_interruptible(wq);
 }

@@ -433,7 +452,7 @@ static void svc_write_space(struct sock *sk)
 		svc_xprt_enqueue(&svsk->sk_xprt);
 	}
 
-	smp_mb();
+	svc_no_smp_mb();
 	if (wq && waitqueue_active(wq)) {
 		dprintk("RPC svc_write_space: someone sleeping on %p\n",
 		       svsk);

@@ -789,7 +808,7 @@ static void svc_tcp_listen_data_ready(struct sock *sk)
 	}
 
 	wq = sk_sleep(sk);
-	smp_mb();
+	svc_no_smp_mb();
 	if (wq && waitqueue_active(wq))
 		wake_up_interruptible_all(wq);
 }

@@ -811,7 +830,7 @@ static void svc_tcp_state_change(struct sock *sk)
 		set_bit(XPT_CLOSE, &svsk->sk_xprt.xpt_flags);
 		svc_xprt_enqueue(&svsk->sk_xprt);
 	}
-	smp_mb();
+	svc_no_smp_mb();
 	if (wq && waitqueue_active(wq))
 		wake_up_interruptible_all(wq);
 }

@@ -827,7 +846,7 @@ static void svc_tcp_data_ready(struct sock *sk)
 		set_bit(XPT_DATA, &svsk->sk_xprt.xpt_flags);
 		svc_xprt_enqueue(&svsk->sk_xprt);
 	}
-	smp_mb();
+	svc_no_smp_mb();
 	if (wq && waitqueue_active(wq))
 		wake_up_interruptible(wq);
 }

@@ -1599,7 +1618,7 @@ static void svc_sock_detach(struct svc_xprt *xprt)
 	sk->sk_write_space = svsk->sk_owspace;
 
 	wq = sk_sleep(sk);
-	smp_mb();
+	svc_no_smp_mb();
 	if (wq && waitqueue_active(wq))
 		wake_up_interruptible(wq);
 }

---
Kosuke TATSUKAWA  | 3rd IT Platform Department
                  | IT Platform Division, NEC Corporation
                  | tatsu-zZGIbrA41Td8UrSeD/g0lQ@public.gmane.org
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