[PATCH net] sctp: fix a success return may hide an error

[PATCH net] sctp: fix a success return may hide an error

[Xin Long]

Now in the end of sctp_outq_flush, sctp calls sctp_packet_transmit
in a loop. The return of current sctp_packet_transmit always covers
the prior one's. If the last call of sctp_packet_transmit return a
success, it may hide the error that returns from the prior call.

This patch is to fix this by keeping the old error until the new
error returns from sctp_packet_transmit. Did TAHI test against this
fix, no regression is found.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
---
 net/sctp/outqueue.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sctp/outqueue.c b/net/sctp/outqueue.c
index 72e54a4..b97c8ad 100644
--- a/net/sctp/outqueue.c
+++ b/net/sctp/outqueue.c
@@ -1193,7 +1193,7 @@ sctp_flush_out:
 						      send_ready);
 		packet = &t->packet;
 		if (!sctp_packet_empty(packet))
-			error = sctp_packet_transmit(packet, gfp);
+			error = sctp_packet_transmit(packet, gfp) ? : error;
 
 		/* Clear the burst limited state, if any */
 		sctp_transport_burst_reset(t);
-- 
2.1.0

[PATCH net] sctp: fix a success return may hide an error

[Xin Long]

Now in the end of sctp_outq_flush, sctp calls sctp_packet_transmit
in a loop. The return of current sctp_packet_transmit always covers
the prior one's. If the last call of sctp_packet_transmit return a
success, it may hide the error that returns from the prior call.

This patch is to fix this by keeping the old error until the new
error returns from sctp_packet_transmit. Did TAHI test against this
fix, no regression is found.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
---
 net/sctp/outqueue.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sctp/outqueue.c b/net/sctp/outqueue.c
index 72e54a4..b97c8ad 100644
--- a/net/sctp/outqueue.c
+++ b/net/sctp/outqueue.c
@@ -1193,7 +1193,7 @@ sctp_flush_out:
 						      send_ready);
 		packet = &t->packet;
 		if (!sctp_packet_empty(packet))
-			error = sctp_packet_transmit(packet, gfp);
+			error = sctp_packet_transmit(packet, gfp) ? : error;
 
 		/* Clear the burst limited state, if any */
 		sctp_transport_burst_reset(t);
-- 
2.1.0

Re: [PATCH net] sctp: fix a success return may hide an error

[Marcelo Ricardo Leitner]

On Thu, Aug 11, 2016 at 08:52:58PM +0800, Xin Long wrote:
> Now in the end of sctp_outq_flush, sctp calls sctp_packet_transmit
> in a loop. The return of current sctp_packet_transmit always covers
> the prior one's. If the last call of sctp_packet_transmit return a
> success, it may hide the error that returns from the prior call.
> 
> This patch is to fix this by keeping the old error until the new
> error returns from sctp_packet_transmit. Did TAHI test against this
> fix, no regression is found.
> 
> Signed-off-by: Xin Long <lucien.xin@gmail.com>

Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>

> ---
>  net/sctp/outqueue.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/sctp/outqueue.c b/net/sctp/outqueue.c
> index 72e54a4..b97c8ad 100644
> --- a/net/sctp/outqueue.c
> +++ b/net/sctp/outqueue.c
> @@ -1193,7 +1193,7 @@ sctp_flush_out:
>  						      send_ready);
>  		packet = &t->packet;
>  		if (!sctp_packet_empty(packet))
> -			error = sctp_packet_transmit(packet, gfp);
> +			error = sctp_packet_transmit(packet, gfp) ? : error;
>  
>  		/* Clear the burst limited state, if any */
>  		sctp_transport_burst_reset(t);
> -- 
> 2.1.0
> 

Re: [PATCH net] sctp: fix a success return may hide an error

[Marcelo Ricardo Leitner]

On Thu, Aug 11, 2016 at 08:52:58PM +0800, Xin Long wrote:
> Now in the end of sctp_outq_flush, sctp calls sctp_packet_transmit
> in a loop. The return of current sctp_packet_transmit always covers
> the prior one's. If the last call of sctp_packet_transmit return a
> success, it may hide the error that returns from the prior call.
> 
> This patch is to fix this by keeping the old error until the new
> error returns from sctp_packet_transmit. Did TAHI test against this
> fix, no regression is found.
> 
> Signed-off-by: Xin Long <lucien.xin@gmail.com>

Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>

> ---
>  net/sctp/outqueue.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/sctp/outqueue.c b/net/sctp/outqueue.c
> index 72e54a4..b97c8ad 100644
> --- a/net/sctp/outqueue.c
> +++ b/net/sctp/outqueue.c
> @@ -1193,7 +1193,7 @@ sctp_flush_out:
>  						      send_ready);
>  		packet = &t->packet;
>  		if (!sctp_packet_empty(packet))
> -			error = sctp_packet_transmit(packet, gfp);
> +			error = sctp_packet_transmit(packet, gfp) ? : error;
>  
>  		/* Clear the burst limited state, if any */
>  		sctp_transport_burst_reset(t);
> -- 
> 2.1.0
> 

Re: [PATCH net] sctp: fix a success return may hide an error

[Neil Horman]

On Thu, Aug 11, 2016 at 08:52:58PM +0800, Xin Long wrote:
> Now in the end of sctp_outq_flush, sctp calls sctp_packet_transmit
> in a loop. The return of current sctp_packet_transmit always covers
> the prior one's. If the last call of sctp_packet_transmit return a
> success, it may hide the error that returns from the prior call.
> 
> This patch is to fix this by keeping the old error until the new
> error returns from sctp_packet_transmit. Did TAHI test against this
> fix, no regression is found.
> 
> Signed-off-by: Xin Long <lucien.xin@gmail.com>
> ---
>  net/sctp/outqueue.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/sctp/outqueue.c b/net/sctp/outqueue.c
> index 72e54a4..b97c8ad 100644
> --- a/net/sctp/outqueue.c
> +++ b/net/sctp/outqueue.c
> @@ -1193,7 +1193,7 @@ sctp_flush_out:
>  						      send_ready);
>  		packet = &t->packet;
>  		if (!sctp_packet_empty(packet))
> -			error = sctp_packet_transmit(packet, gfp);
> +			error = sctp_packet_transmit(packet, gfp) ? : error;
>  
>  		/* Clear the burst limited state, if any */
>  		sctp_transport_burst_reset(t);
> -- 
> 2.1.0
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-sctp" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 
Acked-by: Neil Horman <nhorman@tuxdriver.com>

Re: [PATCH net] sctp: fix a success return may hide an error

[Neil Horman]

On Thu, Aug 11, 2016 at 08:52:58PM +0800, Xin Long wrote:
> Now in the end of sctp_outq_flush, sctp calls sctp_packet_transmit
> in a loop. The return of current sctp_packet_transmit always covers
> the prior one's. If the last call of sctp_packet_transmit return a
> success, it may hide the error that returns from the prior call.
> 
> This patch is to fix this by keeping the old error until the new
> error returns from sctp_packet_transmit. Did TAHI test against this
> fix, no regression is found.
> 
> Signed-off-by: Xin Long <lucien.xin@gmail.com>
> ---
>  net/sctp/outqueue.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/sctp/outqueue.c b/net/sctp/outqueue.c
> index 72e54a4..b97c8ad 100644
> --- a/net/sctp/outqueue.c
> +++ b/net/sctp/outqueue.c
> @@ -1193,7 +1193,7 @@ sctp_flush_out:
>  						      send_ready);
>  		packet = &t->packet;
>  		if (!sctp_packet_empty(packet))
> -			error = sctp_packet_transmit(packet, gfp);
> +			error = sctp_packet_transmit(packet, gfp) ? : error;
>  
>  		/* Clear the burst limited state, if any */
>  		sctp_transport_burst_reset(t);
> -- 
> 2.1.0
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-sctp" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 
Acked-by: Neil Horman <nhorman@tuxdriver.com>

Re: [PATCH net] sctp: fix a success return may hide an error

[David Miller]

From: Xin Long <lucien.xin@gmail.com>
Date: Thu, 11 Aug 2016 20:52:58 +0800

> Now in the end of sctp_outq_flush, sctp calls sctp_packet_transmit
> in a loop. The return of current sctp_packet_transmit always covers
> the prior one's. If the last call of sctp_packet_transmit return a
> success, it may hide the error that returns from the prior call.
> 
> This patch is to fix this by keeping the old error until the new
> error returns from sctp_packet_transmit. Did TAHI test against this
> fix, no regression is found.
> 
> Signed-off-by: Xin Long <lucien.xin@gmail.com>

This style of error handling is dangerous.  The first error can be
lost.

For example, if sctp_outq_flush_rtx() earlier in this function returns
an error, it will be lost if any invocation of the function
sctp_packet_transmit() at the end function signals an error.

I think you should always preserve the first error that is recorded
into 'error'.

I also wonder about why sctp_outq_flush_rtx() errors are completely
ignored and don't influence the control flow here in any way.

Re: [PATCH net] sctp: fix a success return may hide an error

[David Miller]

From: Xin Long <lucien.xin@gmail.com>
Date: Thu, 11 Aug 2016 20:52:58 +0800

> Now in the end of sctp_outq_flush, sctp calls sctp_packet_transmit
> in a loop. The return of current sctp_packet_transmit always covers
> the prior one's. If the last call of sctp_packet_transmit return a
> success, it may hide the error that returns from the prior call.
> 
> This patch is to fix this by keeping the old error until the new
> error returns from sctp_packet_transmit. Did TAHI test against this
> fix, no regression is found.
> 
> Signed-off-by: Xin Long <lucien.xin@gmail.com>

This style of error handling is dangerous.  The first error can be
lost.

For example, if sctp_outq_flush_rtx() earlier in this function returns
an error, it will be lost if any invocation of the function
sctp_packet_transmit() at the end function signals an error.

I think you should always preserve the first error that is recorded
into 'error'.

I also wonder about why sctp_outq_flush_rtx() errors are completely
ignored and don't influence the control flow here in any way.

Re: [PATCH net] sctp: fix a success return may hide an error

[Xin Long]

>
> This style of error handling is dangerous.  The first error can be
> lost.
>
> For example, if sctp_outq_flush_rtx() earlier in this function returns
> an error, it will be lost if any invocation of the function
> sctp_packet_transmit() at the end function signals an error.
>
> I think you should always preserve the first error that is recorded
> into 'error'.
>
> I also wonder about why sctp_outq_flush_rtx() errors are completely
> ignored and don't influence the control flow here in any way.

Yes, the first error can be lost.
Here we just keep the last error. We don't really have to return the
first error or return it on the first failure.

[1]
Both sctp_outq_flush_rtx and sctp_packet_transmit can ONLY
return one error (-ENOMEM), as sctp_outq_flush_rtx also calls
sctp_packet_transmit.

[2]
It's the original codes that it doesn't return immediately when
sctp_outq_flush_rtx returns error. I guess it just doesn't want
to stop flushing out transport_list only because it fail to flush
rtx.
even sctp_packet_transmit_chunk in sctp_outq_flush also just
put the error into sk->sk_err, instread of returning immediately.

So we cannot return the err at the first failure as [2], the error
here is always -ENOMEM as [1].
I think to return the last error here is ok, at least  not dangerous,
can also fix the issue "a success return may hide an error" with
clear codes. :)

Re: [PATCH net] sctp: fix a success return may hide an error

[Xin Long]

>
> This style of error handling is dangerous.  The first error can be
> lost.
>
> For example, if sctp_outq_flush_rtx() earlier in this function returns
> an error, it will be lost if any invocation of the function
> sctp_packet_transmit() at the end function signals an error.
>
> I think you should always preserve the first error that is recorded
> into 'error'.
>
> I also wonder about why sctp_outq_flush_rtx() errors are completely
> ignored and don't influence the control flow here in any way.

Yes, the first error can be lost.
Here we just keep the last error. We don't really have to return the
first error or return it on the first failure.

[1]
Both sctp_outq_flush_rtx and sctp_packet_transmit can ONLY
return one error (-ENOMEM), as sctp_outq_flush_rtx also calls
sctp_packet_transmit.

[2]
It's the original codes that it doesn't return immediately when
sctp_outq_flush_rtx returns error. I guess it just doesn't want
to stop flushing out transport_list only because it fail to flush
rtx.
even sctp_packet_transmit_chunk in sctp_outq_flush also just
put the error into sk->sk_err, instread of returning immediately.

So we cannot return the err at the first failure as [2], the error
here is always -ENOMEM as [1].
I think to return the last error here is ok, at least  not dangerous,
can also fix the issue "a success return may hide an error" with
clear codes. :)

RE: [PATCH net] sctp: fix a success return may hide an error

[David Laight]

From: Xin Long
> Sent: 13 August 2016 08:48
> >
> > This style of error handling is dangerous.  The first error can be
> > lost.
> >
> > For example, if sctp_outq_flush_rtx() earlier in this function returns
> > an error, it will be lost if any invocation of the function
> > sctp_packet_transmit() at the end function signals an error.
> >
> > I think you should always preserve the first error that is recorded
> > into 'error'.
> >
> > I also wonder about why sctp_outq_flush_rtx() errors are completely
> > ignored and don't influence the control flow here in any way.
> 
> Yes, the first error can be lost.
> Here we just keep the last error. We don't really have to return the
> first error or return it on the first failure.
> 
> [1]
> Both sctp_outq_flush_rtx and sctp_packet_transmit can ONLY
> return one error (-ENOMEM), as sctp_outq_flush_rtx also calls
> sctp_packet_transmit.

What is the effect of the error?
If it is 'just' equivalent to a lost ethernet packet (and the skb (etc)
is freed) then the protocol will recover.
If it is anything else then the error path is probably wrong.

Also after one error is it actually worth trying to send anything else
at all? ISTM that the code should either:
1) wait for resources and retry.
2) discard the entire queue (freeing resource) and hope the protocol
   timers will recover.

> [2]
> It's the original codes that it doesn't return immediately when
> sctp_outq_flush_rtx returns error. I guess it just doesn't want
> to stop flushing out transport_list only because it fail to flush
> rtx.
> even sctp_packet_transmit_chunk in sctp_outq_flush also just
> put the error into sk->sk_err, instread of returning immediately.
> 
> So we cannot return the err at the first failure as [2], the error
> here is always -ENOMEM as [1].
> I think to return the last error here is ok, at least  not dangerous,
> can also fix the issue "a success return may hide an error" with
> clear codes. :)

Which code looks at sk->sk_err?
It doesn't look right to be setting an error code on the socket due
a transmit packet discard.

	David

RE: [PATCH net] sctp: fix a success return may hide an error

[David Laight]
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Re: [PATCH net] sctp: fix a success return may hide an error

[Xin Long]

>>
>> [1]
>> Both sctp_outq_flush_rtx and sctp_packet_transmit can ONLY
>> return one error (-ENOMEM), as sctp_outq_flush_rtx also calls
>> sctp_packet_transmit.
>
> What is the effect of the error?
> If it is 'just' equivalent to a lost ethernet packet (and the skb (etc)
> is freed) then the protocol will recover.
> If it is anything else then the error path is probably wrong.
This err returns back to sctp_sendmsg, there sctp will abort asoc.
in this function, sctp tries to do 3 things:
1. flush rtx queue
2. transmit the packet of current transport
3. flush all the transports.
Now sctp would do them one by one, even if one of them returns err.

>
> Also after one error is it actually worth trying to send anything else
> at all? ISTM that the code should either:
yeah, that's the problem.
the "sctp_flush_out:" code tries to force clear all the transport before
returning even if there're errors already.

> 1) wait for resources and retry.
> 2) discard the entire queue (freeing resource) and hope the protocol
>    timers will recover.
It's a different process, will think about it.

>
>> [2]
>> It's the original codes that it doesn't return immediately when
>> sctp_outq_flush_rtx returns error. I guess it just doesn't want
>> to stop flushing out transport_list only because it fail to flush
>> rtx.
>> even sctp_packet_transmit_chunk in sctp_outq_flush also just
>> put the error into sk->sk_err, instread of returning immediately.
>>
>> So we cannot return the err at the first failure as [2], the error
>> here is always -ENOMEM as [1].
>> I think to return the last error here is ok, at least  not dangerous,
>> can also fix the issue "a success return may hide an error" with
>> clear codes. :)
>
> Which code looks at sk->sk_err?
> It doesn't look right to be setting an error code on the socket due
> a transmit packet discard.
I guess sctp_packet_transmit_chunk's return value is used for
'status' (like PMTU_FULL,RWND_FUL... ), that's why err was
put into sk->sk_err.   This err is supposed to be checked in
sctp_sendmsg, but there sctp_error check sk->sk_err only when
err == -EPIPE.
yes, we need to fix this, thanks.

Re: [PATCH net] sctp: fix a success return may hide an error

[Xin Long]

>>
>> [1]
>> Both sctp_outq_flush_rtx and sctp_packet_transmit can ONLY
>> return one error (-ENOMEM), as sctp_outq_flush_rtx also calls
>> sctp_packet_transmit.
>
> What is the effect of the error?
> If it is 'just' equivalent to a lost ethernet packet (and the skb (etc)
> is freed) then the protocol will recover.
> If it is anything else then the error path is probably wrong.
This err returns back to sctp_sendmsg, there sctp will abort asoc.
in this function, sctp tries to do 3 things:
1. flush rtx queue
2. transmit the packet of current transport
3. flush all the transports.
Now sctp would do them one by one, even if one of them returns err.

>
> Also after one error is it actually worth trying to send anything else
> at all? ISTM that the code should either:
yeah, that's the problem.
the "sctp_flush_out:" code tries to force clear all the transport before
returning even if there're errors already.

> 1) wait for resources and retry.
> 2) discard the entire queue (freeing resource) and hope the protocol
>    timers will recover.
It's a different process, will think about it.

>
>> [2]
>> It's the original codes that it doesn't return immediately when
>> sctp_outq_flush_rtx returns error. I guess it just doesn't want
>> to stop flushing out transport_list only because it fail to flush
>> rtx.
>> even sctp_packet_transmit_chunk in sctp_outq_flush also just
>> put the error into sk->sk_err, instread of returning immediately.
>>
>> So we cannot return the err at the first failure as [2], the error
>> here is always -ENOMEM as [1].
>> I think to return the last error here is ok, at least  not dangerous,
>> can also fix the issue "a success return may hide an error" with
>> clear codes. :)
>
> Which code looks at sk->sk_err?
> It doesn't look right to be setting an error code on the socket due
> a transmit packet discard.
I guess sctp_packet_transmit_chunk's return value is used for
'status' (like PMTU_FULL,RWND_FUL... ), that's why err was
put into sk->sk_err.   This err is supposed to be checked in
sctp_sendmsg, but there sctp_error check sk->sk_err only when
err = -EPIPE.
yes, we need to fix this, thanks.

RE: [PATCH net] sctp: fix a success return may hide an error

[David Laight]

From: Xin Long
> Sent: 16 August 2016 12:34
>
> >> Both sctp_outq_flush_rtx and sctp_packet_transmit can ONLY
> >> return one error (-ENOMEM), as sctp_outq_flush_rtx also calls
> >> sctp_packet_transmit.
> >
> > What is the effect of the error?
> > If it is 'just' equivalent to a lost ethernet packet (and the skb (etc)
> > is freed) then the protocol will recover.
> > If it is anything else then the error path is probably wrong.
>
> This err returns back to sctp_sendmsg, there sctp will abort asoc.

That doesn't seem a good idea.
You don't want to abort the association if there is a transient
memory allocation failure.
You also can't drop data chunks.

> in this function, sctp tries to do 3 things:
> 1. flush rtx queue
> 2. transmit the packet of current transport
> 3. flush all the transports.
> Now sctp would do them one by one, even if one of them returns err.

You probably need to explain what 'flush' means here.
I think it means 'process and send', but it might mean 'discard the
contents of'.

Last time I looked at the sctp code my head exploded.
ISTR it is a mess of timing errors waiting to happen
(and I write comms protocol stack code for a living).

	David

RE: [PATCH net] sctp: fix a success return may hide an error

[David Laight]
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Re: [PATCH net] sctp: fix a success return may hide an error

[Marcelo Ricardo Leitner]

On Tue, Aug 16, 2016 at 04:01:50PM +0000, David Laight wrote:
> From: Xin Long
> > Sent: 16 August 2016 12:34
> >
> > >> Both sctp_outq_flush_rtx and sctp_packet_transmit can ONLY
> > >> return one error (-ENOMEM), as sctp_outq_flush_rtx also calls
> > >> sctp_packet_transmit.
> > >
> > > What is the effect of the error?
> > > If it is 'just' equivalent to a lost ethernet packet (and the skb (etc)
> > > is freed) then the protocol will recover.
> > > If it is anything else then the error path is probably wrong.
> >
> > This err returns back to sctp_sendmsg, there sctp will abort asoc.

That's not right I think. sctp_sendmsg will only free the asoc if it was
created to send that specific chunk. And in this case, this change
should have no effect as it can't have sctp_outq_flush() touching
several transports in a row.

I'm basing on:
out_free:
        if (new_asoc)
                sctp_association_free(asoc);

and sctp_recvmsg will just fetch, return and clear the error via
sctp_skb_recv_datagram, but not free it.

Do you see any other place freeing it?

> 
> That doesn't seem a good idea.
> You don't want to abort the association if there is a transient
> memory allocation failure.
> You also can't drop data chunks.

>From a system-wise POV, this behavior - to free the new asoc in case of
transient memory allocation failure - doesn't seem bad to me.
That's what will have to happen if any allocation before it failed and
also it helps the system to reduce the stress a little bit. I don't see
any inconsistency/problems here because we are not dropping a single
random chunk but instead we are actually refusing to initialize a new
asoc in such conditions.

Nevertheless, I agree that letting the application see ENOMEM errors when
the data actually got queued and is being fully handled, as in, it will
be retransmitted later, is not be wise, as the application probably
won't be able to distinguish from ENOMEMs that it should retry or not.
Here I see a problem, yet it's not due to this specific change, perhaps
it just got attention because of it. In this situation, we should handle
ENOMEMs internally if possible so the application can know that if it
hits an ENOMEM, it's real and it has to retry.

Fixing this inconsistency may very well cause us to let that new asoc to
live longer, works for me too.

> 
> > in this function, sctp tries to do 3 things:
> > 1. flush rtx queue
> > 2. transmit the packet of current transport
> > 3. flush all the transports.
> > Now sctp would do them one by one, even if one of them returns err.
> 
> You probably need to explain what 'flush' means here.
> I think it means 'process and send', but it might mean 'discard the
> contents of'.

Yes, the first. He probably use the work 'flush' because the function is
called .._flush_..

> Last time I looked at the sctp code my head exploded.
> ISTR it is a mess of timing errors waiting to happen
> (and I write comms protocol stack code for a living).

Well, it may be, but we are trying to improve it.  Please continue
discussing the fixes so we can keep improving it. :)

  Marcelo

Re: [PATCH net] sctp: fix a success return may hide an error

[Marcelo Ricardo Leitner]

On Tue, Aug 16, 2016 at 04:01:50PM +0000, David Laight wrote:
> From: Xin Long
> > Sent: 16 August 2016 12:34
> >
> > >> Both sctp_outq_flush_rtx and sctp_packet_transmit can ONLY
> > >> return one error (-ENOMEM), as sctp_outq_flush_rtx also calls
> > >> sctp_packet_transmit.
> > >
> > > What is the effect of the error?
> > > If it is 'just' equivalent to a lost ethernet packet (and the skb (etc)
> > > is freed) then the protocol will recover.
> > > If it is anything else then the error path is probably wrong.
> >
> > This err returns back to sctp_sendmsg, there sctp will abort asoc.

That's not right I think. sctp_sendmsg will only free the asoc if it was
created to send that specific chunk. And in this case, this change
should have no effect as it can't have sctp_outq_flush() touching
several transports in a row.

I'm basing on:
out_free:
        if (new_asoc)
                sctp_association_free(asoc);

and sctp_recvmsg will just fetch, return and clear the error via
sctp_skb_recv_datagram, but not free it.

Do you see any other place freeing it?

> 
> That doesn't seem a good idea.
> You don't want to abort the association if there is a transient
> memory allocation failure.
> You also can't drop data chunks.

From a system-wise POV, this behavior - to free the new asoc in case of
transient memory allocation failure - doesn't seem bad to me.
That's what will have to happen if any allocation before it failed and
also it helps the system to reduce the stress a little bit. I don't see
any inconsistency/problems here because we are not dropping a single
random chunk but instead we are actually refusing to initialize a new
asoc in such conditions.

Nevertheless, I agree that letting the application see ENOMEM errors when
the data actually got queued and is being fully handled, as in, it will
be retransmitted later, is not be wise, as the application probably
won't be able to distinguish from ENOMEMs that it should retry or not.
Here I see a problem, yet it's not due to this specific change, perhaps
it just got attention because of it. In this situation, we should handle
ENOMEMs internally if possible so the application can know that if it
hits an ENOMEM, it's real and it has to retry.

Fixing this inconsistency may very well cause us to let that new asoc to
live longer, works for me too.

> 
> > in this function, sctp tries to do 3 things:
> > 1. flush rtx queue
> > 2. transmit the packet of current transport
> > 3. flush all the transports.
> > Now sctp would do them one by one, even if one of them returns err.
> 
> You probably need to explain what 'flush' means here.
> I think it means 'process and send', but it might mean 'discard the
> contents of'.

Yes, the first. He probably use the work 'flush' because the function is
called .._flush_..

> Last time I looked at the sctp code my head exploded.
> ISTR it is a mess of timing errors waiting to happen
> (and I write comms protocol stack code for a living).

Well, it may be, but we are trying to improve it.  Please continue
discussing the fixes so we can keep improving it. :)

  Marcelo

Re: [PATCH net] sctp: fix a success return may hide an error

[Xin Long]

>> > This err returns back to sctp_sendmsg, there sctp will abort asoc.
>
> That's not right I think. sctp_sendmsg will only free the asoc if it was
> created to send that specific chunk. And in this case, this change
> should have no effect as it can't have sctp_outq_flush() touching
> several transports in a row.
>
> I'm basing on:
> out_free:
>         if (new_asoc)
>                 sctp_association_free(asoc);
>
> and sctp_recvmsg will just fetch, return and clear the error via
> sctp_skb_recv_datagram, but not free it.
>
> Do you see any other place freeing it?
Sorry, you are right, it free assoc just for new_asoc.

>
>>
>> That doesn't seem a good idea.
>> You don't want to abort the association if there is a transient
>> memory allocation failure.
>> You also can't drop data chunks.
>
> From a system-wise POV, this behavior - to free the new asoc in case of
> transient memory allocation failure - doesn't seem bad to me.
> That's what will have to happen if any allocation before it failed and
> also it helps the system to reduce the stress a little bit. I don't see
> any inconsistency/problems here because we are not dropping a single
> random chunk but instead we are actually refusing to initialize a new
> asoc in such conditions.
>
> Nevertheless, I agree that letting the application see ENOMEM errors when
> the data actually got queued and is being fully handled, as in, it will
> be retransmitted later, is not be wise, as the application probably
> won't be able to distinguish from ENOMEMs that it should retry or not.
> Here I see a problem, yet it's not due to this specific change, perhaps
> it just got attention because of it. In this situation, we should handle
> ENOMEMs internally if possible so the application can know that if it
> hits an ENOMEM, it's real and it has to retry.
If  letting the application see ENOMEM errors, and sctp has to drop this
chunk, instead of retransmiting the ENOMEM chunk, but the ENOMEM
chunk may not be the chunk from current msg, as it flush all the queue.
even if users get an ENOMEM error, they may re-send a chunk that is same
with the one that is still in retransmit queue.

>
> Fixing this inconsistency may very well cause us to let that new asoc to
> live longer, works for me too.
>
>>
>> > in this function, sctp tries to do 3 things:
>> > 1. flush rtx queue
>> > 2. transmit the packet of current transport
>> > 3. flush all the transports.
>> > Now sctp would do them one by one, even if one of them returns err.
>>
>> You probably need to explain what 'flush' means here.
>> I think it means 'process and send', but it might mean 'discard the
>> contents of'.
>
> Yes, the first. He probably use the work 'flush' because the function is
> called .._flush_..
Yes, :D

>
>> Last time I looked at the sctp code my head exploded.
>> ISTR it is a mess of timing errors waiting to happen
>> (and I write comms protocol stack code for a living).
>
> Well, it may be, but we are trying to improve it.  Please continue
> discussing the fixes so we can keep improving it. :)
>
>   Marcelo
>

Re: [PATCH net] sctp: fix a success return may hide an error

[Xin Long]

>> > This err returns back to sctp_sendmsg, there sctp will abort asoc.
>
> That's not right I think. sctp_sendmsg will only free the asoc if it was
> created to send that specific chunk. And in this case, this change
> should have no effect as it can't have sctp_outq_flush() touching
> several transports in a row.
>
> I'm basing on:
> out_free:
>         if (new_asoc)
>                 sctp_association_free(asoc);
>
> and sctp_recvmsg will just fetch, return and clear the error via
> sctp_skb_recv_datagram, but not free it.
>
> Do you see any other place freeing it?
Sorry, you are right, it free assoc just for new_asoc.

>
>>
>> That doesn't seem a good idea.
>> You don't want to abort the association if there is a transient
>> memory allocation failure.
>> You also can't drop data chunks.
>
> From a system-wise POV, this behavior - to free the new asoc in case of
> transient memory allocation failure - doesn't seem bad to me.
> That's what will have to happen if any allocation before it failed and
> also it helps the system to reduce the stress a little bit. I don't see
> any inconsistency/problems here because we are not dropping a single
> random chunk but instead we are actually refusing to initialize a new
> asoc in such conditions.
>
> Nevertheless, I agree that letting the application see ENOMEM errors when
> the data actually got queued and is being fully handled, as in, it will
> be retransmitted later, is not be wise, as the application probably
> won't be able to distinguish from ENOMEMs that it should retry or not.
> Here I see a problem, yet it's not due to this specific change, perhaps
> it just got attention because of it. In this situation, we should handle
> ENOMEMs internally if possible so the application can know that if it
> hits an ENOMEM, it's real and it has to retry.
If  letting the application see ENOMEM errors, and sctp has to drop this
chunk, instead of retransmiting the ENOMEM chunk, but the ENOMEM
chunk may not be the chunk from current msg, as it flush all the queue.
even if users get an ENOMEM error, they may re-send a chunk that is same
with the one that is still in retransmit queue.

>
> Fixing this inconsistency may very well cause us to let that new asoc to
> live longer, works for me too.
>
>>
>> > in this function, sctp tries to do 3 things:
>> > 1. flush rtx queue
>> > 2. transmit the packet of current transport
>> > 3. flush all the transports.
>> > Now sctp would do them one by one, even if one of them returns err.
>>
>> You probably need to explain what 'flush' means here.
>> I think it means 'process and send', but it might mean 'discard the
>> contents of'.
>
> Yes, the first. He probably use the work 'flush' because the function is
> called .._flush_..
Yes, :D

>
>> Last time I looked at the sctp code my head exploded.
>> ISTR it is a mess of timing errors waiting to happen
>> (and I write comms protocol stack code for a living).
>
> Well, it may be, but we are trying to improve it.  Please continue
> discussing the fixes so we can keep improving it. :)
>
>   Marcelo
>

Re: [PATCH net] sctp: fix a success return may hide an error

[Marcelo Ricardo Leitner]

On Wed, Aug 17, 2016 at 02:24:19AM +0800, Xin Long wrote:
> >> > This err returns back to sctp_sendmsg, there sctp will abort asoc.
> >
> > That's not right I think. sctp_sendmsg will only free the asoc if it was
> > created to send that specific chunk. And in this case, this change
> > should have no effect as it can't have sctp_outq_flush() touching
> > several transports in a row.
> >
> > I'm basing on:
> > out_free:
> >         if (new_asoc)
> >                 sctp_association_free(asoc);
> >
> > and sctp_recvmsg will just fetch, return and clear the error via
> > sctp_skb_recv_datagram, but not free it.
> >
> > Do you see any other place freeing it?
> Sorry, you are right, it free assoc just for new_asoc.
> 
> >
> >>
> >> That doesn't seem a good idea.
> >> You don't want to abort the association if there is a transient
> >> memory allocation failure.
> >> You also can't drop data chunks.
> >
> > From a system-wise POV, this behavior - to free the new asoc in case of
> > transient memory allocation failure - doesn't seem bad to me.
> > That's what will have to happen if any allocation before it failed and
> > also it helps the system to reduce the stress a little bit. I don't see
> > any inconsistency/problems here because we are not dropping a single
> > random chunk but instead we are actually refusing to initialize a new
> > asoc in such conditions.
> >
> > Nevertheless, I agree that letting the application see ENOMEM errors when
> > the data actually got queued and is being fully handled, as in, it will
> > be retransmitted later, is not be wise, as the application probably
> > won't be able to distinguish from ENOMEMs that it should retry or not.
> > Here I see a problem, yet it's not due to this specific change, perhaps
> > it just got attention because of it. In this situation, we should handle
> > ENOMEMs internally if possible so the application can know that if it
> > hits an ENOMEM, it's real and it has to retry.
> If  letting the application see ENOMEM errors, and sctp has to drop this
> chunk, instead of retransmiting the ENOMEM chunk, but the ENOMEM
> chunk may not be the chunk from current msg, as it flush all the queue.
> even if users get an ENOMEM error, they may re-send a chunk that is same
> with the one that is still in retransmit queue.

Yep, one more reason to handle those internally when safe.

Re: [PATCH net] sctp: fix a success return may hide an error

[Marcelo Ricardo Leitner]

On Wed, Aug 17, 2016 at 02:24:19AM +0800, Xin Long wrote:
> >> > This err returns back to sctp_sendmsg, there sctp will abort asoc.
> >
> > That's not right I think. sctp_sendmsg will only free the asoc if it was
> > created to send that specific chunk. And in this case, this change
> > should have no effect as it can't have sctp_outq_flush() touching
> > several transports in a row.
> >
> > I'm basing on:
> > out_free:
> >         if (new_asoc)
> >                 sctp_association_free(asoc);
> >
> > and sctp_recvmsg will just fetch, return and clear the error via
> > sctp_skb_recv_datagram, but not free it.
> >
> > Do you see any other place freeing it?
> Sorry, you are right, it free assoc just for new_asoc.
> 
> >
> >>
> >> That doesn't seem a good idea.
> >> You don't want to abort the association if there is a transient
> >> memory allocation failure.
> >> You also can't drop data chunks.
> >
> > From a system-wise POV, this behavior - to free the new asoc in case of
> > transient memory allocation failure - doesn't seem bad to me.
> > That's what will have to happen if any allocation before it failed and
> > also it helps the system to reduce the stress a little bit. I don't see
> > any inconsistency/problems here because we are not dropping a single
> > random chunk but instead we are actually refusing to initialize a new
> > asoc in such conditions.
> >
> > Nevertheless, I agree that letting the application see ENOMEM errors when
> > the data actually got queued and is being fully handled, as in, it will
> > be retransmitted later, is not be wise, as the application probably
> > won't be able to distinguish from ENOMEMs that it should retry or not.
> > Here I see a problem, yet it's not due to this specific change, perhaps
> > it just got attention because of it. In this situation, we should handle
> > ENOMEMs internally if possible so the application can know that if it
> > hits an ENOMEM, it's real and it has to retry.
> If  letting the application see ENOMEM errors, and sctp has to drop this
> chunk, instead of retransmiting the ENOMEM chunk, but the ENOMEM
> chunk may not be the chunk from current msg, as it flush all the queue.
> even if users get an ENOMEM error, they may re-send a chunk that is same
> with the one that is still in retransmit queue.

Yep, one more reason to handle those internally when safe.

Re: [PATCH net] sctp: fix a success return may hide an error

[Marcelo Ricardo Leitner]

On Tue, Aug 16, 2016 at 03:33:30PM -0300, Marcelo Ricardo Leitner wrote:
> On Wed, Aug 17, 2016 at 02:24:19AM +0800, Xin Long wrote:
> > >> > This err returns back to sctp_sendmsg, there sctp will abort asoc.
> > >
> > > That's not right I think. sctp_sendmsg will only free the asoc if it was
> > > created to send that specific chunk. And in this case, this change
> > > should have no effect as it can't have sctp_outq_flush() touching
> > > several transports in a row.
> > >
> > > I'm basing on:
> > > out_free:
> > >         if (new_asoc)
> > >                 sctp_association_free(asoc);
> > >
> > > and sctp_recvmsg will just fetch, return and clear the error via
> > > sctp_skb_recv_datagram, but not free it.
> > >
> > > Do you see any other place freeing it?
> > Sorry, you are right, it free assoc just for new_asoc.
> > 
> > >
> > >>
> > >> That doesn't seem a good idea.
> > >> You don't want to abort the association if there is a transient
> > >> memory allocation failure.
> > >> You also can't drop data chunks.
> > >
> > > From a system-wise POV, this behavior - to free the new asoc in case of
> > > transient memory allocation failure - doesn't seem bad to me.
> > > That's what will have to happen if any allocation before it failed and
> > > also it helps the system to reduce the stress a little bit. I don't see
> > > any inconsistency/problems here because we are not dropping a single
> > > random chunk but instead we are actually refusing to initialize a new
> > > asoc in such conditions.
> > >
> > > Nevertheless, I agree that letting the application see ENOMEM errors when
> > > the data actually got queued and is being fully handled, as in, it will
> > > be retransmitted later, is not be wise, as the application probably
> > > won't be able to distinguish from ENOMEMs that it should retry or not.
> > > Here I see a problem, yet it's not due to this specific change, perhaps
> > > it just got attention because of it. In this situation, we should handle
> > > ENOMEMs internally if possible so the application can know that if it
> > > hits an ENOMEM, it's real and it has to retry.
> > If  letting the application see ENOMEM errors, and sctp has to drop this
> > chunk, instead of retransmiting the ENOMEM chunk, but the ENOMEM
> > chunk may not be the chunk from current msg, as it flush all the queue.
> > even if users get an ENOMEM error, they may re-send a chunk that is same
> > with the one that is still in retransmit queue.
> 
> Yep, one more reason to handle those internally when safe.

Xin, maybe you can squash this patch and this ENOMEM handling? I'm
thinking that handling ENOMEM may result in similar situations in other
places, so we have a common reasoning on them.

Re: [PATCH net] sctp: fix a success return may hide an error

[Marcelo Ricardo Leitner]

On Tue, Aug 16, 2016 at 03:33:30PM -0300, Marcelo Ricardo Leitner wrote:
> On Wed, Aug 17, 2016 at 02:24:19AM +0800, Xin Long wrote:
> > >> > This err returns back to sctp_sendmsg, there sctp will abort asoc.
> > >
> > > That's not right I think. sctp_sendmsg will only free the asoc if it was
> > > created to send that specific chunk. And in this case, this change
> > > should have no effect as it can't have sctp_outq_flush() touching
> > > several transports in a row.
> > >
> > > I'm basing on:
> > > out_free:
> > >         if (new_asoc)
> > >                 sctp_association_free(asoc);
> > >
> > > and sctp_recvmsg will just fetch, return and clear the error via
> > > sctp_skb_recv_datagram, but not free it.
> > >
> > > Do you see any other place freeing it?
> > Sorry, you are right, it free assoc just for new_asoc.
> > 
> > >
> > >>
> > >> That doesn't seem a good idea.
> > >> You don't want to abort the association if there is a transient
> > >> memory allocation failure.
> > >> You also can't drop data chunks.
> > >
> > > From a system-wise POV, this behavior - to free the new asoc in case of
> > > transient memory allocation failure - doesn't seem bad to me.
> > > That's what will have to happen if any allocation before it failed and
> > > also it helps the system to reduce the stress a little bit. I don't see
> > > any inconsistency/problems here because we are not dropping a single
> > > random chunk but instead we are actually refusing to initialize a new
> > > asoc in such conditions.
> > >
> > > Nevertheless, I agree that letting the application see ENOMEM errors when
> > > the data actually got queued and is being fully handled, as in, it will
> > > be retransmitted later, is not be wise, as the application probably
> > > won't be able to distinguish from ENOMEMs that it should retry or not.
> > > Here I see a problem, yet it's not due to this specific change, perhaps
> > > it just got attention because of it. In this situation, we should handle
> > > ENOMEMs internally if possible so the application can know that if it
> > > hits an ENOMEM, it's real and it has to retry.
> > If  letting the application see ENOMEM errors, and sctp has to drop this
> > chunk, instead of retransmiting the ENOMEM chunk, but the ENOMEM
> > chunk may not be the chunk from current msg, as it flush all the queue.
> > even if users get an ENOMEM error, they may re-send a chunk that is same
> > with the one that is still in retransmit queue.
> 
> Yep, one more reason to handle those internally when safe.

Xin, maybe you can squash this patch and this ENOMEM handling? I'm
thinking that handling ENOMEM may result in similar situations in other
places, so we have a common reasoning on them.

RE: [PATCH net] sctp: fix a success return may hide an error

[David Laight]

From: Marcelo Ricardo Leitner
> Sent: 16 August 2016 18:25
...
> > That doesn't seem a good idea.
> > You don't want to abort the association if there is a transient
> > memory allocation failure.
> > You also can't drop data chunks.
> 
> From a system-wise POV, this behavior - to free the new asoc in case of
> transient memory allocation failure - doesn't seem bad to me.
> That's what will have to happen if any allocation before it failed and
> also it helps the system to reduce the stress a little bit. I don't see
> any inconsistency/problems here because we are not dropping a single
> random chunk but instead we are actually refusing to initialize a new
> asoc in such conditions.

Failing a new association should be ok, whether purists will like
connect() failing ENOMEM is another matter.

> Nevertheless, I agree that letting the application see ENOMEM errors when
> the data actually got queued and is being fully handled, as in, it will
> be retransmitted later, is not be wise, as the application probably
> won't be able to distinguish from ENOMEMs that it should retry or not.

I think an application would be justified in thinking that an ENOMEM return
meant that the system call had no effect.

For send() even ENOMEM is really wrong, it should be treated as 'flow control'
and either block or return EAGAIN/EWOULDBLOCK.
Getting POLLOUT set is left as an exercise to the reader :-)

...
> Well, it may be, but we are trying to improve it.  Please continue
> discussing the fixes so we can keep improving it. :)

Indeed, we have customers who use sctp (for M3UA).
We don't do anything 'complicated', but do end up sending a lot of short
data chunks.

	David

Re: [PATCH net] sctp: fix a success return may hide an error

[Xin Long]

>> > If  letting the application see ENOMEM errors, and sctp has to drop this
>> > chunk, instead of retransmiting the ENOMEM chunk, but the ENOMEM
>> > chunk may not be the chunk from current msg, as it flush all the queue.
>> > even if users get an ENOMEM error, they may re-send a chunk that is same
>> > with the one that is still in retransmit queue.
>>
>> Yep, one more reason to handle those internally when safe.
I just checked tcp_sendmsg, it doesn't return any transmit error to user,
*NOT ONLY* ENOMEM.  you can check __tcp_push_pending_frames
and tcp_push, their return type is even void. although it may get
err from sk->sk_err:
    err = sk_stream_error(sk, flags, err);
But I didn't see it put any err into sk->sk_err in the main transmit
path.

yes, tcp_write_xmit has return value, as well as tcp_transmit_skb and
err = icsk->icsk_af_ops->queue_xmit(sk, skb, &inet->cork.fl). but
all of them are just used for internal, never return to userspace

In tcp_write_xmit, it even uses "unlikely':
if (unlikely(tcp_transmit_skb(sk, skb, 1, gfp)))
       break;


>
> Xin, maybe you can squash this patch and this ENOMEM handling? I'm
> thinking that handling ENOMEM may result in similar situations in other
> places, so we have a common reasoning on them.
>
So this reason does really matter, and not only for ENOMEM in transmit
path.

Re: [PATCH net] sctp: fix a success return may hide an error

[Xin Long]

>> > If  letting the application see ENOMEM errors, and sctp has to drop this
>> > chunk, instead of retransmiting the ENOMEM chunk, but the ENOMEM
>> > chunk may not be the chunk from current msg, as it flush all the queue.
>> > even if users get an ENOMEM error, they may re-send a chunk that is same
>> > with the one that is still in retransmit queue.
>>
>> Yep, one more reason to handle those internally when safe.
I just checked tcp_sendmsg, it doesn't return any transmit error to user,
*NOT ONLY* ENOMEM.  you can check __tcp_push_pending_frames
and tcp_push, their return type is even void. although it may get
err from sk->sk_err:
    err = sk_stream_error(sk, flags, err);
But I didn't see it put any err into sk->sk_err in the main transmit
path.

yes, tcp_write_xmit has return value, as well as tcp_transmit_skb and
err = icsk->icsk_af_ops->queue_xmit(sk, skb, &inet->cork.fl). but
all of them are just used for internal, never return to userspace

In tcp_write_xmit, it even uses "unlikely':
if (unlikely(tcp_transmit_skb(sk, skb, 1, gfp)))
       break;


>
> Xin, maybe you can squash this patch and this ENOMEM handling? I'm
> thinking that handling ENOMEM may result in similar situations in other
> places, so we have a common reasoning on them.
>
So this reason does really matter, and not only for ENOMEM in transmit
path.

Re: [PATCH net] sctp: fix a success return may hide an error

['Marcelo Ricardo Leitner']

On Wed, Aug 17, 2016 at 09:01:38AM +0000, David Laight wrote:
> From: Marcelo Ricardo Leitner
> > Sent: 16 August 2016 18:25
> ...
> > > That doesn't seem a good idea.
> > > You don't want to abort the association if there is a transient
> > > memory allocation failure.
> > > You also can't drop data chunks.
> > 
> > From a system-wise POV, this behavior - to free the new asoc in case of
> > transient memory allocation failure - doesn't seem bad to me.
> > That's what will have to happen if any allocation before it failed and
> > also it helps the system to reduce the stress a little bit. I don't see
> > any inconsistency/problems here because we are not dropping a single
> > random chunk but instead we are actually refusing to initialize a new
> > asoc in such conditions.
> 
> Failing a new association should be ok, whether purists will like
> connect() failing ENOMEM is another matter.
> 

Good point.

> > Nevertheless, I agree that letting the application see ENOMEM errors when
> > the data actually got queued and is being fully handled, as in, it will
> > be retransmitted later, is not be wise, as the application probably
> > won't be able to distinguish from ENOMEMs that it should retry or not.
> 
> I think an application would be justified in thinking that an ENOMEM return
> meant that the system call had no effect.
> 

Yep

> For send() even ENOMEM is really wrong, it should be treated as 'flow control'
> and either block or return EAGAIN/EWOULDBLOCK.

Agreed.

> Getting POLLOUT set is left as an exercise to the reader :-)
> 

:-)

> ...
> > Well, it may be, but we are trying to improve it.  Please continue
> > discussing the fixes so we can keep improving it. :)
> 
> Indeed, we have customers who use sctp (for M3UA).
> We don't do anything 'complicated', but do end up sending a lot of short
> data chunks.
> 
> 	David
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-sctp" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 

Re: [PATCH net] sctp: fix a success return may hide an error

['Marcelo Ricardo Leitner']

On Wed, Aug 17, 2016 at 09:01:38AM +0000, David Laight wrote:
> From: Marcelo Ricardo Leitner
> > Sent: 16 August 2016 18:25
> ...
> > > That doesn't seem a good idea.
> > > You don't want to abort the association if there is a transient
> > > memory allocation failure.
> > > You also can't drop data chunks.
> > 
> > From a system-wise POV, this behavior - to free the new asoc in case of
> > transient memory allocation failure - doesn't seem bad to me.
> > That's what will have to happen if any allocation before it failed and
> > also it helps the system to reduce the stress a little bit. I don't see
> > any inconsistency/problems here because we are not dropping a single
> > random chunk but instead we are actually refusing to initialize a new
> > asoc in such conditions.
> 
> Failing a new association should be ok, whether purists will like
> connect() failing ENOMEM is another matter.
> 

Good point.

> > Nevertheless, I agree that letting the application see ENOMEM errors when
> > the data actually got queued and is being fully handled, as in, it will
> > be retransmitted later, is not be wise, as the application probably
> > won't be able to distinguish from ENOMEMs that it should retry or not.
> 
> I think an application would be justified in thinking that an ENOMEM return
> meant that the system call had no effect.
> 

Yep

> For send() even ENOMEM is really wrong, it should be treated as 'flow control'
> and either block or return EAGAIN/EWOULDBLOCK.

Agreed.

> Getting POLLOUT set is left as an exercise to the reader :-)
> 

:-)

> ...
> > Well, it may be, but we are trying to improve it.  Please continue
> > discussing the fixes so we can keep improving it. :)
> 
> Indeed, we have customers who use sctp (for M3UA).
> We don't do anything 'complicated', but do end up sending a lot of short
> data chunks.
> 
> 	David
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-sctp" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>