socket poll related cleanups

socket poll related cleanups

[Christoph Hellwig]

A couple of cleanups I stumbled upon when studying the networking
poll code.

[PATCH 1/5] net: remove bogus RCU annotations on socket.wq

[Christoph Hellwig]

We never use RCU protection for it, just a lot of cargo-cult
rcu_deference_protects calls.

Note that we do keep the kfree_rcu call for it, as the references through
struct sock are RCU protected and thus might require a grace period before
freeing.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 include/linux/net.h |  2 +-
 include/net/sock.h  |  2 +-
 net/socket.c        | 10 ++++------
 3 files changed, 6 insertions(+), 8 deletions(-)

diff --git a/include/linux/net.h b/include/linux/net.h
index 6554d3ba4396..e0930678c8bf 100644
--- a/include/linux/net.h
+++ b/include/linux/net.h
@@ -114,7 +114,7 @@ struct socket {
 
 	unsigned long		flags;
 
-	struct socket_wq __rcu	*wq;
+	struct socket_wq	*wq;
 
 	struct file		*file;
 	struct sock		*sk;
diff --git a/include/net/sock.h b/include/net/sock.h
index b3b75419eafe..ad85d37c83c8 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1725,7 +1725,7 @@ static inline void sock_graft(struct sock *sk, struct socket *parent)
 {
 	WARN_ON(parent->sk);
 	write_lock_bh(&sk->sk_callback_lock);
-	sk->sk_wq = parent->wq;
+	rcu_assign_pointer(sk->sk_wq, parent->wq);
 	parent->sk = sk;
 	sk_set_socket(sk, parent);
 	sk->sk_uid = SOCK_INODE(parent)->i_uid;
diff --git a/net/socket.c b/net/socket.c
index 85633622c94d..39e0afbdd797 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -251,7 +251,7 @@ static struct inode *sock_alloc_inode(struct super_block *sb)
 	init_waitqueue_head(&wq->wait);
 	wq->fasync_list = NULL;
 	wq->flags = 0;
-	RCU_INIT_POINTER(ei->socket.wq, wq);
+	ei->socket.wq = wq;
 
 	ei->socket.state = SS_UNCONNECTED;
 	ei->socket.flags = 0;
@@ -265,11 +265,9 @@ static struct inode *sock_alloc_inode(struct super_block *sb)
 static void sock_destroy_inode(struct inode *inode)
 {
 	struct socket_alloc *ei;
-	struct socket_wq *wq;
 
 	ei = container_of(inode, struct socket_alloc, vfs_inode);
-	wq = rcu_dereference_protected(ei->socket.wq, 1);
-	kfree_rcu(wq, rcu);
+	kfree_rcu(ei->socket.wq, rcu);
 	kmem_cache_free(sock_inode_cachep, ei);
 }
 
@@ -603,7 +601,7 @@ static void __sock_release(struct socket *sock, struct inode *inode)
 		module_put(owner);
 	}
 
-	if (rcu_dereference_protected(sock->wq, 1)->fasync_list)
+	if (sock->wq->fasync_list)
 		pr_err("%s: fasync list not empty!\n", __func__);
 
 	if (!sock->file) {
@@ -1172,7 +1170,7 @@ static int sock_fasync(int fd, struct file *filp, int on)
 		return -EINVAL;
 
 	lock_sock(sk);
-	wq = rcu_dereference_protected(sock->wq, lockdep_sock_is_held(sk));
+	wq = sock->wq;
 	fasync_helper(fd, filp, on, &wq->fasync_list);
 
 	if (!wq->fasync_list)
-- 
2.18.0

Re: [PATCH 1/5] net: remove bogus RCU annotations on socket.wq

[David Miller]

From: Christoph Hellwig <hch@lst.de>
Date: Fri, 27 Jul 2018 16:02:10 +0200

> We never use RCU protection for it, just a lot of cargo-cult
> rcu_deference_protects calls.
> 
> Note that we do keep the kfree_rcu call for it, as the references through
> struct sock are RCU protected and thus might require a grace period before
> freeing.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

These were added by Eric Dumazet and I would never accuse him of cargo
cult programming.

All of the rcu_dereference_protects() calls are legit, even though some
of them use '1' as the protects condition because in fact we know the
object is dead and gone through an RCU cycle at that point.

Let's skip this for now.  The rest of your series looks fine so why
don't you resubmit this series with just #2-#5?

Thanks.

Re: [PATCH 1/5] net: remove bogus RCU annotations on socket.wq

[Christoph Hellwig]

On Sun, Jul 29, 2018 at 01:05:51PM -0700, David Miller wrote:
> From: Christoph Hellwig <hch@lst.de>
> Date: Fri, 27 Jul 2018 16:02:10 +0200
> 
> > We never use RCU protection for it, just a lot of cargo-cult
> > rcu_deference_protects calls.
> > 
> > Note that we do keep the kfree_rcu call for it, as the references through
> > struct sock are RCU protected and thus might require a grace period before
> > freeing.
> > 
> > Signed-off-by: Christoph Hellwig <hch@lst.de>
> 
> These were added by Eric Dumazet and I would never accuse him of cargo
> cult programming.
> 
> All of the rcu_dereference_protects() calls are legit, even though some
> of them use '1' as the protects condition because in fact we know the
> object is dead and gone through an RCU cycle at that point.

I disagree, but I'll resend it the patch with Eric and Paul in CC to
settle the argument.

Re: [PATCH 1/5] net: remove bogus RCU annotations on socket.wq

[David Miller]

From: Christoph Hellwig <hch@lst.de>
Date: Mon, 30 Jul 2018 09:44:54 +0200

> On Sun, Jul 29, 2018 at 01:05:51PM -0700, David Miller wrote:
>> From: Christoph Hellwig <hch@lst.de>
>> Date: Fri, 27 Jul 2018 16:02:10 +0200
>> 
>> > We never use RCU protection for it, just a lot of cargo-cult
>> > rcu_deference_protects calls.
>> > 
>> > Note that we do keep the kfree_rcu call for it, as the references through
>> > struct sock are RCU protected and thus might require a grace period before
>> > freeing.
>> > 
>> > Signed-off-by: Christoph Hellwig <hch@lst.de>
>> 
>> These were added by Eric Dumazet and I would never accuse him of cargo
>> cult programming.
>> 
>> All of the rcu_dereference_protects() calls are legit, even though some
>> of them use '1' as the protects condition because in fact we know the
>> object is dead and gone through an RCU cycle at that point.
> 
> I disagree, but I'll resend it the patch with Eric and Paul in CC to
> settle the argument.

Thank you.

[PATCH 2/5] net: simplify sock_poll_wait

[Christoph Hellwig]

The wait_address argument is always directly derived from the filp
argument, so remove it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 crypto/af_alg.c        |  2 +-
 include/net/sock.h     | 11 ++++++-----
 net/atm/common.c       |  2 +-
 net/caif/caif_socket.c |  2 +-
 net/core/datagram.c    |  2 +-
 net/dccp/proto.c       |  2 +-
 net/ipv4/tcp.c         |  2 +-
 net/iucv/af_iucv.c     |  2 +-
 net/nfc/llcp_sock.c    |  2 +-
 net/rxrpc/af_rxrpc.c   |  2 +-
 net/smc/af_smc.c       |  2 +-
 net/tipc/socket.c      |  2 +-
 net/unix/af_unix.c     |  4 ++--
 13 files changed, 19 insertions(+), 18 deletions(-)

diff --git a/crypto/af_alg.c b/crypto/af_alg.c
index c166f424871c..b053179e0bc5 100644
--- a/crypto/af_alg.c
+++ b/crypto/af_alg.c
@@ -1071,7 +1071,7 @@ __poll_t af_alg_poll(struct file *file, struct socket *sock,
 	struct af_alg_ctx *ctx = ask->private;
 	__poll_t mask;
 
-	sock_poll_wait(file, sk_sleep(sk), wait);
+	sock_poll_wait(file, wait);
 	mask = 0;
 
 	if (!ctx->more || ctx->used)
diff --git a/include/net/sock.h b/include/net/sock.h
index ad85d37c83c8..946ee8651714 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1994,16 +1994,17 @@ static inline bool skwq_has_sleeper(struct socket_wq *wq)
 /**
  * sock_poll_wait - place memory barrier behind the poll_wait call.
  * @filp:           file
- * @wait_address:   socket wait queue
  * @p:              poll_table
  *
  * See the comments in the wq_has_sleeper function.
  */
-static inline void sock_poll_wait(struct file *filp,
-		wait_queue_head_t *wait_address, poll_table *p)
+static inline void sock_poll_wait(struct file *filp, poll_table *p)
 {
-	if (!poll_does_not_wait(p) && wait_address) {
-		poll_wait(filp, wait_address, p);
+	struct socket *sock = filp->private_data;
+	wait_queue_head_t *wq = sk_sleep(sock->sk);
+
+	if (!poll_does_not_wait(p) && wq) {
+		poll_wait(filp, wq, p);
 		/* We need to be sure we are in sync with the
 		 * socket flags modification.
 		 *
diff --git a/net/atm/common.c b/net/atm/common.c
index a7a68e509628..9f8cb0d2e71e 100644
--- a/net/atm/common.c
+++ b/net/atm/common.c
@@ -653,7 +653,7 @@ __poll_t vcc_poll(struct file *file, struct socket *sock, poll_table *wait)
 	struct atm_vcc *vcc;
 	__poll_t mask;
 
-	sock_poll_wait(file, sk_sleep(sk), wait);
+	sock_poll_wait(file, wait);
 	mask = 0;
 
 	vcc = ATM_SD(sock);
diff --git a/net/caif/caif_socket.c b/net/caif/caif_socket.c
index a6fb1b3bcad9..d18965f3291f 100644
--- a/net/caif/caif_socket.c
+++ b/net/caif/caif_socket.c
@@ -941,7 +941,7 @@ static __poll_t caif_poll(struct file *file,
 	__poll_t mask;
 	struct caifsock *cf_sk = container_of(sk, struct caifsock, sk);
 
-	sock_poll_wait(file, sk_sleep(sk), wait);
+	sock_poll_wait(file, wait);
 	mask = 0;
 
 	/* exceptional events? */
diff --git a/net/core/datagram.c b/net/core/datagram.c
index 9938952c5c78..9aac0d63d53e 100644
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -837,7 +837,7 @@ __poll_t datagram_poll(struct file *file, struct socket *sock,
 	struct sock *sk = sock->sk;
 	__poll_t mask;
 
-	sock_poll_wait(file, sk_sleep(sk), wait);
+	sock_poll_wait(file, wait);
 	mask = 0;
 
 	/* exceptional events? */
diff --git a/net/dccp/proto.c b/net/dccp/proto.c
index 0d56e36a6db7..875858c8b059 100644
--- a/net/dccp/proto.c
+++ b/net/dccp/proto.c
@@ -325,7 +325,7 @@ __poll_t dccp_poll(struct file *file, struct socket *sock,
 	__poll_t mask;
 	struct sock *sk = sock->sk;
 
-	sock_poll_wait(file, sk_sleep(sk), wait);
+	sock_poll_wait(file, wait);
 	if (sk->sk_state == DCCP_LISTEN)
 		return inet_csk_listen_poll(sk);
 
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 4491faf83f4f..0b93290c9255 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -507,7 +507,7 @@ __poll_t tcp_poll(struct file *file, struct socket *sock, poll_table *wait)
 	const struct tcp_sock *tp = tcp_sk(sk);
 	int state;
 
-	sock_poll_wait(file, sk_sleep(sk), wait);
+	sock_poll_wait(file, wait);
 
 	state = inet_sk_state_load(sk);
 	if (state == TCP_LISTEN)
diff --git a/net/iucv/af_iucv.c b/net/iucv/af_iucv.c
index 893a022f9620..e7b93cd14b52 100644
--- a/net/iucv/af_iucv.c
+++ b/net/iucv/af_iucv.c
@@ -1494,7 +1494,7 @@ __poll_t iucv_sock_poll(struct file *file, struct socket *sock,
 	struct sock *sk = sock->sk;
 	__poll_t mask = 0;
 
-	sock_poll_wait(file, sk_sleep(sk), wait);
+	sock_poll_wait(file, wait);
 
 	if (sk->sk_state == IUCV_LISTEN)
 		return iucv_accept_poll(sk);
diff --git a/net/nfc/llcp_sock.c b/net/nfc/llcp_sock.c
index ea0c0c6f1874..dd4adf8b1167 100644
--- a/net/nfc/llcp_sock.c
+++ b/net/nfc/llcp_sock.c
@@ -556,7 +556,7 @@ static __poll_t llcp_sock_poll(struct file *file, struct socket *sock,
 
 	pr_debug("%p\n", sk);
 
-	sock_poll_wait(file, sk_sleep(sk), wait);
+	sock_poll_wait(file, wait);
 
 	if (sk->sk_state == LLCP_LISTEN)
 		return llcp_accept_poll(sk);
diff --git a/net/rxrpc/af_rxrpc.c b/net/rxrpc/af_rxrpc.c
index 2b463047dd7b..ac44d8afffb1 100644
--- a/net/rxrpc/af_rxrpc.c
+++ b/net/rxrpc/af_rxrpc.c
@@ -741,7 +741,7 @@ static __poll_t rxrpc_poll(struct file *file, struct socket *sock,
 	struct rxrpc_sock *rx = rxrpc_sk(sk);
 	__poll_t mask;
 
-	sock_poll_wait(file, sk_sleep(sk), wait);
+	sock_poll_wait(file, wait);
 	mask = 0;
 
 	/* the socket is readable if there are any messages waiting on the Rx
diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
index 05e4ffe5aabd..09df79313f60 100644
--- a/net/smc/af_smc.c
+++ b/net/smc/af_smc.c
@@ -1351,7 +1351,7 @@ static __poll_t smc_poll(struct file *file, struct socket *sock,
 			mask |= EPOLLERR;
 	} else {
 		if (sk->sk_state != SMC_CLOSED)
-			sock_poll_wait(file, sk_sleep(sk), wait);
+			sock_poll_wait(file, wait);
 		if (sk->sk_err)
 			mask |= EPOLLERR;
 		if ((sk->sk_shutdown == SHUTDOWN_MASK) ||
diff --git a/net/tipc/socket.c b/net/tipc/socket.c
index 930852c54d7a..9a9050131724 100644
--- a/net/tipc/socket.c
+++ b/net/tipc/socket.c
@@ -716,7 +716,7 @@ static __poll_t tipc_poll(struct file *file, struct socket *sock,
 	struct tipc_sock *tsk = tipc_sk(sk);
 	__poll_t revents = 0;
 
-	sock_poll_wait(file, sk_sleep(sk), wait);
+	sock_poll_wait(file, wait);
 
 	if (sk->sk_shutdown & RCV_SHUTDOWN)
 		revents |= EPOLLRDHUP | EPOLLIN | EPOLLRDNORM;
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index e5473c03d667..1772a0e32665 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -2635,7 +2635,7 @@ static __poll_t unix_poll(struct file *file, struct socket *sock, poll_table *wa
 	struct sock *sk = sock->sk;
 	__poll_t mask;
 
-	sock_poll_wait(file, sk_sleep(sk), wait);
+	sock_poll_wait(file, wait);
 	mask = 0;
 
 	/* exceptional events? */
@@ -2672,7 +2672,7 @@ static __poll_t unix_dgram_poll(struct file *file, struct socket *sock,
 	unsigned int writable;
 	__poll_t mask;
 
-	sock_poll_wait(file, sk_sleep(sk), wait);
+	sock_poll_wait(file, wait);
 	mask = 0;
 
 	/* exceptional events? */
-- 
2.18.0

[PATCH 3/5] net: don not detour through struct sock to find the poll waitqueue

[Christoph Hellwig]

For any open socket file descriptor sock->sk->sk_wq->wait will always
point to sock->wq->wait.  That means we can do the shorter dereference
and removal a NULL check and don't have to not worry about any RCU
protection.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 include/net/sock.h | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index 946ee8651714..9b6011912691 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -2001,10 +2001,9 @@ static inline bool skwq_has_sleeper(struct socket_wq *wq)
 static inline void sock_poll_wait(struct file *filp, poll_table *p)
 {
 	struct socket *sock = filp->private_data;
-	wait_queue_head_t *wq = sk_sleep(sock->sk);
 
-	if (!poll_does_not_wait(p) && wq) {
-		poll_wait(filp, wq, p);
+	if (!poll_does_not_wait(p)) {
+		poll_wait(filp, &sock->wq->wait, p);
 		/* We need to be sure we are in sync with the
 		 * socket flags modification.
 		 *
-- 
2.18.0

[PATCH 4/5] net: remove sock_poll_busy_loop

[Christoph Hellwig]

There is no point in hiding this logic in a helper.  Also remove the
useless events != 0 check and only busy loop once we know we actually
have a poll method.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 include/net/busy_poll.h | 9 ---------
 net/socket.c            | 5 ++++-
 2 files changed, 4 insertions(+), 10 deletions(-)

diff --git a/include/net/busy_poll.h b/include/net/busy_poll.h
index c5187438af38..25d762cf47f2 100644
--- a/include/net/busy_poll.h
+++ b/include/net/busy_poll.h
@@ -121,15 +121,6 @@ static inline void sk_busy_loop(struct sock *sk, int nonblock)
 #endif
 }
 
-static inline void sock_poll_busy_loop(struct socket *sock, __poll_t events)
-{
-	if (sk_can_busy_loop(sock->sk) &&
-	    events && (events & POLL_BUSY_LOOP)) {
-		/* once, only if requested by syscall */
-		sk_busy_loop(sock->sk, 1);
-	}
-}
-
 /* if this socket can poll_ll, tell the system call */
 static inline __poll_t sock_poll_busy_flag(struct socket *sock)
 {
diff --git a/net/socket.c b/net/socket.c
index 39e0afbdd797..399d2ccec89d 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -1130,9 +1130,12 @@ static __poll_t sock_poll(struct file *file, poll_table *wait)
 	struct socket *sock = file->private_data;
 	__poll_t events = poll_requested_events(wait);
 
-	sock_poll_busy_loop(sock, events);
 	if (!sock->ops->poll)
 		return 0;
+
+	/* poll once if requested by the syscall */
+	if (sk_can_busy_loop(sock->sk) && (events & POLL_BUSY_LOOP))
+		sk_busy_loop(sock->sk, 1);
 	return sock->ops->poll(file, sock, wait) | sock_poll_busy_flag(sock);
 }
 
-- 
2.18.0

[PATCH 5/5] net: remove sock_poll_busy_flag

[Christoph Hellwig]

Fold it into the only caller to make the code simpler and easier to read.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 include/net/busy_poll.h |  6 ------
 net/socket.c            | 16 +++++++++++-----
 2 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/include/net/busy_poll.h b/include/net/busy_poll.h
index 25d762cf47f2..71c72a939bf8 100644
--- a/include/net/busy_poll.h
+++ b/include/net/busy_poll.h
@@ -121,12 +121,6 @@ static inline void sk_busy_loop(struct sock *sk, int nonblock)
 #endif
 }
 
-/* if this socket can poll_ll, tell the system call */
-static inline __poll_t sock_poll_busy_flag(struct socket *sock)
-{
-	return sk_can_busy_loop(sock->sk) ? POLL_BUSY_LOOP : 0;
-}
-
 /* used in the NIC receive handler to mark the skb */
 static inline void skb_mark_napi_id(struct sk_buff *skb,
 				    struct napi_struct *napi)
diff --git a/net/socket.c b/net/socket.c
index 399d2ccec89d..475247e347ae 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -1128,15 +1128,21 @@ EXPORT_SYMBOL(sock_create_lite);
 static __poll_t sock_poll(struct file *file, poll_table *wait)
 {
 	struct socket *sock = file->private_data;
-	__poll_t events = poll_requested_events(wait);
+	__poll_t events = poll_requested_events(wait), flag = 0;
 
 	if (!sock->ops->poll)
 		return 0;
 
-	/* poll once if requested by the syscall */
-	if (sk_can_busy_loop(sock->sk) && (events & POLL_BUSY_LOOP))
-		sk_busy_loop(sock->sk, 1);
-	return sock->ops->poll(file, sock, wait) | sock_poll_busy_flag(sock);
+	if (sk_can_busy_loop(sock->sk)) {
+		/* poll once if requested by the syscall */
+		if (events & POLL_BUSY_LOOP)
+			sk_busy_loop(sock->sk, 1);
+
+		/* if this socket can poll_ll, tell the system call */
+		flag = POLL_BUSY_LOOP;
+	}
+
+	return sock->ops->poll(file, sock, wait) | flag;
 }
 
 static int sock_mmap(struct file *file, struct vm_area_struct *vma)
-- 
2.18.0