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

From: "J. Bruce Fields" <[email-masked]>
Date: Fri, 23 Oct 2015 16:49:45 -0400

On Fri, Oct 23, 2015 at 04:14:10AM +0000, Kosuke Tatsukawa wrote:
> J. Bruce Fields wrote:
> > On Fri, Oct 16, 2015 at 02:28:10AM +0000, Kosuke Tatsukawa wrote:
> >> Tatsukawa Kosuke wrote:
> >> > J. Bruce Fields wrote:
> >> >> On Thu, Oct 15, 2015 at 11:44:20AM +0000, Kosuke Tatsukawa wrote:
> >> >>> Tatsukawa Kosuke wrote:
> >> >>> > J. Bruce Fields wrote:
> >> >>> >> 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.
> > 
> > So is it true that every caller of these socket callbacks has adequate
> > memory barriers between the time the change is made visible and the time
> > the callback is called?
> > 
> > If so, then there's nothing really specific about nfsd here.
> > 
> > In that case maybe it's the networking code that use some documentation,
> > if it doesn't already?  (Or maybe common helper functions for this
> > 
> > 	if (waitqueue_active(wq))
> > 		wake_up(wq)
> > 
> > pattern?)
> 
> Some of the other places defining these callback functions are using
>   static inline bool wq_has_sleeper(struct socket_wq *wq)
> defined in include/net/sock.h
> 
> The comment above the function explains that it was introduced for
> exactly this purpose.
> 
> Even thought the argument variable uses the same name "wq", it has a
> different type from the wq used in svcsock.c (struct socket_wq *
> vs. wait_queue_head_t *).

OK, thanks.  So, I guess it still sounds like the code is OK as is, but
maybe my comment wasn't.  Here's another attempt.

--b.

commit b805ca58a81a
Author: Kosuke Tatsukawa <tatsu-zZGIbrA41Td8UrSeD/[email-masked]>
Date:   Fri Oct 9 01:44:07 2015 +0000

    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.
    
    I found this issue when I was 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).
    
    Signed-off-by: Kosuke Tatsukawa <tatsu-zZGIbrA41Td8UrSeD/[email-masked]>
    [[email-masked],[email-masked]: document instead of adding barriers]
    Signed-off-by: J. Bruce Fields <[email-masked]>

diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
index 48923730722d..1f71eece04d3 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;
+	/*
+	 * Kosuke Tatsukawa points out 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);
 }
 
@@ -1594,7 +1619,7 @@ 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);
 }
 
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