Davide Libenzi a écrit :

On Thu, 25 Jun 2009, Oleg Nesterov wrote:

quoted

Can't really comment this patch, except this all looks reasonable to me.
Add more CCs.

While this can work, IMO it'd be cleaner to have the smp_mb() moved from 
fs/select.c to the ->poll() function.
Having a barrier that matches another one in another susbsystem, because 
of the special locking logic of such subsystem, is not too shiny IMHO.

Yes but barrier is necessary only if add_wait_queue() was actually called, and __pollwait()
does this call.

Adding a plain smp_mb() in tcp_poll() for example would slowdown select()/poll() with NULL
timeout.

Adding a cond test before smp_mb() in tcp_poll() (and other ->poll() functions)
would be litle bit overkill too...

I believe this race was not existent in the past because spin_unlock() had a memory barrier,
and we changed this to a plain memory write at some point...

Most add_wait_queue() calls are followed by a call to set_current_state()
so a proper smp_mb() is explicitly included.

quoted

On 06/25, Jiri Olsa wrote:

quoted

Adding memory barrier to the __pollwait function paired with
receive callbacks.  The smp_mb__after_lock define is added,
since {read|write|spin}_lock() on x86 are full memory barriers.

The race fires, when following code paths meet, and the tp->rcv_nxt and
__add_wait_queue updates stay in CPU caches.


CPU1                         CPU2

sys_select                   receive packet
  ...                        ...
  __add_wait_queue           update tp->rcv_nxt
  ...                        ...
  tp->rcv_nxt check          sock_def_readable
  ...                        {
  schedule                      ...
                                if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
                                        wake_up_interruptible(sk->sk_sleep)
                                ...
                             }

If there was no cache the code would work ok, since the wait_queue and
rcv_nxt are opposit to each other.

Meaning that once tp->rcv_nxt is updated by CPU2, the CPU1 either already
passed the tp->rcv_nxt check and sleeps, or will get the new value for
tp->rcv_nxt and will return with new data mask.
In both cases the process (CPU1) is being added to the wait queue, so the
waitqueue_active (CPU2) call cannot miss and will wake up CPU1.

The bad case is when the __add_wait_queue changes done by CPU1 stay in its
cache, and so does the tp->rcv_nxt update on CPU2 side.  The CPU1 will then
endup calling schedule and sleep forever if there are no more data on the
socket.

wbr,
jirka


Signed-off-by: Jiri Olsa <redacted>

---
 arch/x86/include/asm/spinlock.h |    3 +++
 fs/select.c                     |    4 ++++
 include/linux/spinlock.h        |    5 +++++
 include/net/sock.h              |   18 ++++++++++++++++++
 net/atm/common.c                |    4 ++--
 net/core/sock.c                 |    8 ++++----
 net/dccp/output.c               |    2 +-
 net/iucv/af_iucv.c              |    2 +-
 net/rxrpc/af_rxrpc.c            |    2 +-
 net/unix/af_unix.c              |    2 +-
 10 files changed, 40 insertions(+), 10 deletions(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index b7e5db8..39ecc5f 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h

@@ -302,4 +302,7 @@ static inline void __raw_write_unlock(raw_rwlock_t *rw)
 #define _raw_read_relax(lock)	cpu_relax()
 #define _raw_write_relax(lock)	cpu_relax()

+/* The {read|write|spin}_lock() on x86 are full memory barriers. */
+#define smp_mb__after_lock() do { } while (0)
+
 #endif /* _ASM_X86_SPINLOCK_H */

diff --git a/fs/select.c b/fs/select.c
index d870237..c4bd5f0 100644
--- a/fs/select.c
+++ b/fs/select.c

@@ -219,6 +219,10 @@ static void __pollwait(struct file *filp, wait_queue_head_t *wait_address,
 	init_waitqueue_func_entry(&entry->wait, pollwake);
 	entry->wait.private = pwq;
 	add_wait_queue(wait_address, &entry->wait);
+
+	/* This memory barrier is paired with the smp_mb__after_lock
+	 * in the sk_has_sleeper. */
+	smp_mb();
 }

 int poll_schedule_timeout(struct poll_wqueues *pwq, int state,

diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
index 252b245..ae053bd 100644
--- a/include/linux/spinlock.h
+++ b/include/linux/spinlock.h

@@ -132,6 +132,11 @@ do {								\
 #endif /*__raw_spin_is_contended*/
 #endif

+/* The lock does not imply full memory barrier. */
+#ifndef smp_mb__after_lock
+#define smp_mb__after_lock() smp_mb()
+#endif
+
 /**
  * spin_unlock_wait - wait until the spinlock gets unlocked
  * @lock: the spinlock in question.

diff --git a/include/net/sock.h b/include/net/sock.h
index 352f06b..7fbb143 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h

@@ -1241,6 +1241,24 @@ static inline int sk_has_allocations(const struct sock *sk)
 	return sk_wmem_alloc_get(sk) || sk_rmem_alloc_get(sk);
 }

+/**
+ * sk_has_sleeper - check if there are any waiting processes
+ * @sk: socket
+ *
+ * Returns true if socket has waiting processes
+ */
+static inline int sk_has_sleeper(struct sock *sk)
+{
+	/*
+	 * We need to be sure we are in sync with the
+	 * add_wait_queue modifications to the wait queue.
+	 *
+	 * This memory barrier is paired in the __pollwait.
+	 */
+	smp_mb__after_lock();
+	return sk->sk_sleep && waitqueue_active(sk->sk_sleep);
+}
+
 /*
  * 	Queue a received datagram if it will fit. Stream and sequenced
  *	protocols can't normally use this as they need to fit buffers in

diff --git a/net/atm/common.c b/net/atm/common.c
index c1c9793..67a8642 100644
--- a/net/atm/common.c
+++ b/net/atm/common.c

@@ -92,7 +92,7 @@ static void vcc_sock_destruct(struct sock *sk)
 static void vcc_def_wakeup(struct sock *sk)
 {
 	read_lock(&sk->sk_callback_lock);
-	if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
+	if (sk_has_sleeper(sk))
 		wake_up(sk->sk_sleep);
 	read_unlock(&sk->sk_callback_lock);
 }

@@ -110,7 +110,7 @@ static void vcc_write_space(struct sock *sk)
 	read_lock(&sk->sk_callback_lock);

 	if (vcc_writable(sk)) {
-		if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
+		if (sk_has_sleeper(sk))
 			wake_up_interruptible(sk->sk_sleep);

 		sk_wake_async(sk, SOCK_WAKE_SPACE, POLL_OUT);

diff --git a/net/core/sock.c b/net/core/sock.c
index b0ba569..6354863 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c

@@ -1715,7 +1715,7 @@ EXPORT_SYMBOL(sock_no_sendpage);
 static void sock_def_wakeup(struct sock *sk)
 {
 	read_lock(&sk->sk_callback_lock);
-	if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
+	if (sk_has_sleeper(sk))
 		wake_up_interruptible_all(sk->sk_sleep);
 	read_unlock(&sk->sk_callback_lock);
 }

@@ -1723,7 +1723,7 @@ static void sock_def_wakeup(struct sock *sk)
 static void sock_def_error_report(struct sock *sk)
 {
 	read_lock(&sk->sk_callback_lock);
-	if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
+	if (sk_has_sleeper(sk))
 		wake_up_interruptible_poll(sk->sk_sleep, POLLERR);
 	sk_wake_async(sk, SOCK_WAKE_IO, POLL_ERR);
 	read_unlock(&sk->sk_callback_lock);

@@ -1732,7 +1732,7 @@ static void sock_def_error_report(struct sock *sk)
 static void sock_def_readable(struct sock *sk, int len)
 {
 	read_lock(&sk->sk_callback_lock);
-	if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
+	if (sk_has_sleeper(sk))
 		wake_up_interruptible_sync_poll(sk->sk_sleep, POLLIN |
 						POLLRDNORM | POLLRDBAND);
 	sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_IN);

@@ -1747,7 +1747,7 @@ static void sock_def_write_space(struct sock *sk)
 	 * progress.  --DaveM
 	 */
 	if ((atomic_read(&sk->sk_wmem_alloc) << 1) <= sk->sk_sndbuf) {
-		if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
+		if (sk_has_sleeper(sk))
 			wake_up_interruptible_sync_poll(sk->sk_sleep, POLLOUT |
 						POLLWRNORM | POLLWRBAND);

diff --git a/net/dccp/output.c b/net/dccp/output.c
index c0e88c1..c96119f 100644
--- a/net/dccp/output.c
+++ b/net/dccp/output.c

@@ -196,7 +196,7 @@ void dccp_write_space(struct sock *sk)
 {
 	read_lock(&sk->sk_callback_lock);

-	if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
+	if (sk_has_sleeper(sk))
 		wake_up_interruptible(sk->sk_sleep);
 	/* Should agree with poll, otherwise some programs break */
 	if (sock_writeable(sk))

diff --git a/net/iucv/af_iucv.c b/net/iucv/af_iucv.c
index 6be5f92..ba0149d 100644
--- a/net/iucv/af_iucv.c
+++ b/net/iucv/af_iucv.c

@@ -306,7 +306,7 @@ static inline int iucv_below_msglim(struct sock *sk)
 static void iucv_sock_wake_msglim(struct sock *sk)
 {
 	read_lock(&sk->sk_callback_lock);
-	if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
+	if (sk_has_sleeper(sk))
 		wake_up_interruptible_all(sk->sk_sleep);
 	sk_wake_async(sk, SOCK_WAKE_SPACE, POLL_OUT);
 	read_unlock(&sk->sk_callback_lock);

diff --git a/net/rxrpc/af_rxrpc.c b/net/rxrpc/af_rxrpc.c
index eac5e7b..60e0e38 100644
--- a/net/rxrpc/af_rxrpc.c
+++ b/net/rxrpc/af_rxrpc.c

@@ -63,7 +63,7 @@ static void rxrpc_write_space(struct sock *sk)
 	_enter("%p", sk);
 	read_lock(&sk->sk_callback_lock);
 	if (rxrpc_writable(sk)) {
-		if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
+		if (sk_has_sleeper(sk))
 			wake_up_interruptible(sk->sk_sleep);
 		sk_wake_async(sk, SOCK_WAKE_SPACE, POLL_OUT);
 	}

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 36d4e44..143143a 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c

@@ -315,7 +315,7 @@ static void unix_write_space(struct sock *sk)
 {
 	read_lock(&sk->sk_callback_lock);
 	if (unix_writable(sk)) {
-		if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
+		if (sk_has_sleeper(sk))
 			wake_up_interruptible_sync(sk->sk_sleep);
 		sk_wake_async(sk, SOCK_WAKE_SPACE, POLL_OUT);
 	}

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- Davide