The __skb_recv_datagram routine in core/ datagram.c provides a general
skb reception factility supposed to be utilized by protocol modules
providing datagram sockets. It encompasses both the actual recvmsg code
and a surrounding 'sleep until data is available' loop. This is
inconvenient if a protocol module has to use additional locking in order
to maintain some per-socket state the generic datagram socket code is
unaware of (as the af_unix code does). The patch below moves the recvmsg
proper code into a new __skb_try_recv_datagram routine which doesn't
sleep and renames wait_for_more_packets to
__skb_wait_for_more_packets, both routines being exported interfaces. The
original __skb_recv_datagram routine is reimplemented on top of these
two functions such that its user-visible behaviour remains unchanged.
Signed-Off-By: Rainer Weikusat <[email protected]>
---
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 4355129..dce91ac 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2788,6 +2788,12 @@ static inline void skb_frag_list_init(struct sk_buff *skb)
#define skb_walk_frags(skb, iter) \
for (iter = skb_shinfo(skb)->frag_list; iter; iter = iter->next)
+
+int __skb_wait_for_more_packets(struct sock *sk, int *err, long *timeo_p,
+ const struct sk_buff *skb);
+struct sk_buff *__skb_try_recv_datagram(struct sock *sk, unsigned flags,
+ int *peeked, int *off, int *err,
+ struct sk_buff **last);
struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned flags,
int *peeked, int *off, int *err);
struct sk_buff *skb_recv_datagram(struct sock *sk, unsigned flags, int noblock,
diff --git a/net/core/datagram.c b/net/core/datagram.c
index d62af69..7daff66 100644
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -83,8 +83,8 @@ static int receiver_wake_function(wait_queue_t *wait, unsigned int mode, int syn
/*
* Wait for the last received packet to be different from skb
*/
-static int wait_for_more_packets(struct sock *sk, int *err, long *timeo_p,
- const struct sk_buff *skb)
+int __skb_wait_for_more_packets(struct sock *sk, int *err, long *timeo_p,
+ const struct sk_buff *skb)
{
int error;
DEFINE_WAIT_FUNC(wait, receiver_wake_function);
@@ -130,6 +130,7 @@ out_noerr:
error = 1;
goto out;
}
+EXPORT_SYMBOL(__skb_wait_for_more_packets);
static struct sk_buff *skb_set_peeked(struct sk_buff *skb)
{
@@ -161,13 +162,15 @@ done:
}
/**
- * __skb_recv_datagram - Receive a datagram skbuff
+ * __skb_try_recv_datagram - Receive a datagram skbuff
* @sk: socket
* @flags: MSG_ flags
* @peeked: returns non-zero if this packet has been seen before
* @off: an offset in bytes to peek skb from. Returns an offset
* within an skb where data actually starts
* @err: error code returned
+ * @last: set to last peeked message to inform the wait function
+ * what to look for when peeking
*
* Get a datagram skbuff, understands the peeking, nonblocking wakeups
* and possible races. This replaces identical code in packet, raw and
@@ -175,9 +178,11 @@ done:
* the long standing peek and read race for datagram sockets. If you
* alter this routine remember it must be re-entrant.
*
- * This function will lock the socket if a skb is returned, so the caller
- * needs to unlock the socket in that case (usually by calling
- * skb_free_datagram)
+ * This function will lock the socket if a skb is returned, so
+ * the caller needs to unlock the socket in that case (usually by
+ * calling skb_free_datagram). Returns NULL with *err set to
+ * -EAGAIN if no data was available or to some other value if an
+ * error was detected.
*
* * It does not lock socket since today. This function is
* * free of race conditions. This measure should/can improve
@@ -191,13 +196,13 @@ done:
* quite explicitly by POSIX 1003.1g, don't change them without having
* the standard around please.
*/
-struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,
- int *peeked, int *off, int *err)
+struct sk_buff *__skb_try_recv_datagram(struct sock *sk, unsigned int flags,
+ int *peeked, int *off, int *err,
+ struct sk_buff **last)
{
struct sk_buff_head *queue = &sk->sk_receive_queue;
- struct sk_buff *skb, *last;
+ struct sk_buff *skb;
unsigned long cpu_flags;
- long timeo;
/*
* Caller is allowed not to check sk->sk_err before skb_recv_datagram()
*/
@@ -206,8 +211,6 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,
if (error)
goto no_packet;
- timeo = sock_rcvtimeo(sk, flags & MSG_DONTWAIT);
-
do {
/* Again only user level code calls this function, so nothing
* interrupt level will suddenly eat the receive_queue.
@@ -217,10 +220,10 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,
*/
int _off = *off;
- last = (struct sk_buff *)queue;
+ *last = (struct sk_buff *)queue;
spin_lock_irqsave(&queue->lock, cpu_flags);
skb_queue_walk(queue, skb) {
- last = skb;
+ *last = skb;
*peeked = skb->peeked;
if (flags & MSG_PEEK) {
if (_off >= skb->len && (skb->len || _off ||
@@ -231,8 +234,11 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,
skb = skb_set_peeked(skb);
error = PTR_ERR(skb);
- if (IS_ERR(skb))
- goto unlock_err;
+ if (IS_ERR(skb)) {
+ spin_unlock_irqrestore(&queue->lock,
+ cpu_flags);
+ goto no_packet;
+ }
atomic_inc(&skb->users);
} else
@@ -242,25 +248,38 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,
*off = _off;
return skb;
}
+
spin_unlock_irqrestore(&queue->lock, cpu_flags);
+ } while (sk_can_busy_loop(sk) &&
+ sk_busy_loop(sk, flags & MSG_DONTWAIT));
- if (sk_can_busy_loop(sk) &&
- sk_busy_loop(sk, flags & MSG_DONTWAIT))
- continue;
+ error = -EAGAIN;
- /* User doesn't want to wait */
- error = -EAGAIN;
- if (!timeo)
- goto no_packet;
+no_packet:
+ *err = error;
+ return NULL;
+}
+EXPORT_SYMBOL(__skb_try_recv_datagram);
- } while (!wait_for_more_packets(sk, err, &timeo, last));
+struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,
+ int *peeked, int *off, int *err)
+{
+ struct sk_buff *skb, *last;
+ long timeo;
- return NULL;
+ timeo = sock_rcvtimeo(sk, flags & MSG_DONTWAIT);
+
+ do {
+ skb = __skb_try_recv_datagram(sk, flags, peeked, off, err,
+ &last);
+ if (skb)
+ return skb;
+
+ if (*err != EAGAIN)
+ break;
+ } while (timeo &&
+ !__skb_wait_for_more_packets(sk, err, &timeo, last));
-unlock_err:
- spin_unlock_irqrestore(&queue->lock, cpu_flags);
-no_packet:
- *err = error;
return NULL;
}
EXPORT_SYMBOL(__skb_recv_datagram);
From: Rainer Weikusat <[email protected]>
Date: Sun, 06 Dec 2015 21:11:34 +0000
> The __skb_recv_datagram routine in core/ datagram.c provides a general
> skb reception factility supposed to be utilized by protocol modules
> providing datagram sockets. It encompasses both the actual recvmsg code
> and a surrounding 'sleep until data is available' loop. This is
> inconvenient if a protocol module has to use additional locking in order
> to maintain some per-socket state the generic datagram socket code is
> unaware of (as the af_unix code does). The patch below moves the recvmsg
> proper code into a new __skb_try_recv_datagram routine which doesn't
> sleep and renames wait_for_more_packets to
> __skb_wait_for_more_packets, both routines being exported interfaces. The
> original __skb_recv_datagram routine is reimplemented on top of these
> two functions such that its user-visible behaviour remains unchanged.
>
> Signed-Off-By: Rainer Weikusat <[email protected]>
Applied to net-next.
David Miller <[email protected]> writes:
> From: Rainer Weikusat <[email protected]>
> Date: Sun, 06 Dec 2015 21:11:34 +0000
>
>> The __skb_recv_datagram routine in core/ datagram.c provides a general
>> skb reception factility supposed to be utilized by protocol modules
>> providing datagram sockets. It encompasses both the actual recvmsg code
>> and a surrounding 'sleep until data is available' loop. This is
>> inconvenient if a protocol module has to use additional locking in order
>> to maintain some per-socket state the generic datagram socket code is
>> unaware of (as the af_unix code does). The patch below moves the recvmsg
>> proper code into a new __skb_try_recv_datagram routine which doesn't
>> sleep and renames wait_for_more_packets to
>> __skb_wait_for_more_packets, both routines being exported interfaces. The
>> original __skb_recv_datagram routine is reimplemented on top of these
>> two functions such that its user-visible behaviour remains unchanged.
>>
>> Signed-Off-By: Rainer Weikusat <[email protected]>
>
> Applied to net-next.
Because of an oversight, this change breaks blocking datagram reception
for anyone but the AF_UNIX SOCK_DGRAM code. I've noticed this by chance
while running a test program for SOCK_STREAM sockets. The problem seems
to be (fix not yet tested) that a - is missing in the *err check in
__skb_recv_datagram.
Proposed fix:
------------
diff --git a/net/core/datagram.c b/net/core/datagram.c
index 7daff66..fa9dc64 100644
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -275,7 +275,7 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,
if (skb)
return skb;
- if (*err != EAGAIN)
+ if (*err != -EAGAIN)
break;
} while (timeo &&
!__skb_wait_for_more_packets(sk, err, &timeo, last));
-------------
Provided this works (very likely), I'll send a real patch for that ASAP.
As the kernel generally uses negated error numbers, *err needs to be
compared with -EAGAIN (d'oh).
Signed-off-by: Rainer Weikusat <[email protected]>
Fixes: ea3793ee29d3
---
diff --git a/net/core/datagram.c b/net/core/datagram.c
index 7daff66..fa9dc64 100644
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -275,7 +275,7 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,
if (skb)
return skb;
- if (*err != EAGAIN)
+ if (*err != -EAGAIN)
break;
} while (timeo &&
!__skb_wait_for_more_packets(sk, err, &timeo, last));
From: Rainer Weikusat <[email protected]>
Date: Mon, 07 Dec 2015 23:30:58 +0000
> As the kernel generally uses negated error numbers, *err needs to be
> compared with -EAGAIN (d'oh).
>
> Signed-off-by: Rainer Weikusat <[email protected]>
> Fixes: ea3793ee29d3
Improperly formatted Fixes: tag, you must also include the commit
header line, in parenthesis and double quotes, after the SHA_ID.
Futhermore this is the wrong SHA_ID.
David Miller <[email protected]> writes:
> From: Rainer Weikusat <[email protected]>
> Date: Mon, 07 Dec 2015 23:30:58 +0000
>
>> As the kernel generally uses negated error numbers, *err needs to be
>> compared with -EAGAIN (d'oh).
>>
>> Signed-off-by: Rainer Weikusat <[email protected]>
>> Fixes: ea3793ee29d3
>
> Improperly formatted Fixes: tag, you must also include the commit
> header line, in parenthesis and double quotes, after the SHA_ID.
>
> Futhermore this is the wrong SHA_ID.
https://git.kernel.org/cgit/linux/kernel/git/davem/net-next.git/commit/?id=ea3793ee29d3
displays the commit I was referring to, namely, the one containing this
+ timeo = sock_rcvtimeo(sk, flags & MSG_DONTWAIT);
+
+ do {
+ skb = __skb_try_recv_datagram(sk, flags, peeked, off, err,
+ &last);
+ if (skb)
+ return skb;
+
+ if (*err != EAGAIN)
+ break;
+ } while (timeo &&
+ !__skb_wait_for_more_packets(sk, err, &timeo, last));
which added the inverted test, IOW, if this is the wrong hash, I have no
idea what the right one could be. I'll resubmit this with the 'one line
summary' added. After noticing the issue around 23:10 UK time yesterday,
I was in a bit of a hurry and stopped reading the 'Fixes' text in
SubmittingPatches after the "with the first 12 characters".
As the kernel generally uses negated error numbers, *err needs to be
compared with -EAGAIN (d'oh).
Signed-off-by: Rainer Weikusat <[email protected]>
Fixes: ea3793ee29d3 ("core: enable more fine-grained datagram reception control")
---
diff --git a/net/core/datagram.c b/net/core/datagram.c
index 7daff66..fa9dc64 100644
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -275,7 +275,7 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,
if (skb)
return skb;
- if (*err != EAGAIN)
+ if (*err != -EAGAIN)
break;
} while (timeo &&
!__skb_wait_for_more_packets(sk, err, &timeo, last));
From: Rainer Weikusat <[email protected]>
Date: Tue, 08 Dec 2015 14:46:36 +0000
> David Miller <[email protected]> writes:
>> From: Rainer Weikusat <[email protected]>
>> Date: Mon, 07 Dec 2015 23:30:58 +0000
>>
>>> As the kernel generally uses negated error numbers, *err needs to be
>>> compared with -EAGAIN (d'oh).
>>>
>>> Signed-off-by: Rainer Weikusat <[email protected]>
>>> Fixes: ea3793ee29d3
>>
>> Improperly formatted Fixes: tag, you must also include the commit
>> header line, in parenthesis and double quotes, after the SHA_ID.
>>
>> Futhermore this is the wrong SHA_ID.
>
> https://git.kernel.org/cgit/linux/kernel/git/davem/net-next.git/commit/?id=ea3793ee29d3
>
> displays the commit I was referring to, namely, the one containing this
My bad.
From: Rainer Weikusat <[email protected]>
Date: Tue, 08 Dec 2015 14:47:56 +0000
> As the kernel generally uses negated error numbers, *err needs to be
> compared with -EAGAIN (d'oh).
>
> Signed-off-by: Rainer Weikusat <[email protected]>
> Fixes: ea3793ee29d3 ("core: enable more fine-grained datagram reception control")
Applied, thanks.
Please, in the future, place proper subsystem prefixes in your
Subject lines. In this case, "net: " would have been appropriate
and it wouldn't be the end of the world if you capitalized
your commit header line too.
I fixed both of these things while installing your change.
Thanks.
David Miller <[email protected]> writes:
[...]
> Please, in the future, place proper subsystem prefixes in your
> Subject lines. In this case, "net: " would have been appropriate
> and it wouldn't be the end of the world if you capitalized
> your commit header line too.
>
> I fixed both of these things while installing your change.
Sorry. Will try to keep this in mind.