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@xxxxxxxxxxxxx>
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@xxxxxxxxxxxxx>
[bfields@xxxxxxxxxx,nfbrown@xxxxxxxxxx: document instead of adding barriers]
Signed-off-by: J. Bruce Fields <bfields@xxxxxxxxxx>
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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