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authorJ. Bruce Fields <bfields@redhat.com>2015-10-09 01:44:07 +0000
committerJ. Bruce Fields <bfields@redhat.com>2015-11-10 17:02:47 -0500
commit0442f14b15f8e7a8b3778a9f8cf640ef89b2df26 (patch)
treedd7cae613d5d154bd945b9c140d4764074bbbe08 /net
parent7fc0564e3a8d16df096f48c9c6425ba84d945c6e (diff)
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>
Diffstat (limited to 'net')
-rw-r--r--net/sunrpc/svcsock.c37
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);
}