Re: [PATCH] [doc] msg_zerocopy.rst: clarify the TCP shutdown scenario

[Frederik Deweerdt <frederik.deweerdt@gmail.com>]

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Hi Willem,

On Wed, Jun 01, 2022 at 09:24:32AM -0400, Willem de Bruijn wrote:
> On Tue, May 31, 2022 at 10:48 PM Frederik Deweerdt
> <frederik.deweerdt@gmail.com> wrote:
> >
> > Hi folks,
> >
> > Based on my understanding, retransmissions of zero copied buffers can
> > happen after `close(2)`, the patch below amends the docs to suggest how
> > notifications should be handled in that case.
> 
> Not just retransmissions. The first transmission similarly may be queued.
> 
> >
[...]
> > @@ -144,6 +144,10 @@ the socket. A socket that has an error queued would normally block
> >  other operations until the error is read. Zerocopy notifications have
> >  a zero error code, however, to not block send and recv calls.
> >
> > +For protocols like TCP, where retransmissions can occur after the
> > +application is done with a given connection, applications should signal
> > +the close to the peer via shutdown(2), and keep polling the error queue
> > +until all transmissions have completed.
> 
> A socket must not be closed until all completion notifications have
> been received.
> 
> Calling shutdown is an optional step. It may be sufficient to simply
> delay close.

Thank you for the feedback, that helps!

What do you think of the attached patch?

Frederik

Signed-off-by: Frederik Deweerdt <frederik.deweerdt@gmail.com>

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commit 3218d973b68bc6d9f88d9e2374f3ada3df5ee7ff
Author: Frederik Deweerdt <frederik.deweerdt@gmail.com>
Date:   Tue May 31 18:23:54 2022 -0700

    [doc] msg_zerocopy.rst: clarify the TCP shutdown scenario
    
    Explicitly mention that applications shouldn't be calling `close(2)` on
    a TCP socket without draining the error queue.

diff --git a/Documentation/networking/msg_zerocopy.rst b/Documentation/networking/msg_zerocopy.rst
index 15920db8d35d..9373631d0a82 100644
--- a/Documentation/networking/msg_zerocopy.rst
+++ b/Documentation/networking/msg_zerocopy.rst
@@ -144,6 +144,11 @@ the socket. A socket that has an error queued would normally block
 other operations until the error is read. Zerocopy notifications have
 a zero error code, however, to not block send and recv calls.
 
+For protocols like TCP, transmissions can occur after the application
+has called close(2). In cases where it's undesirable to delay calling
+close(2) until all notifications have been processed, the application
+can use shutdown(2), and keep polling the error queue until all
+transmissions have completed.
 
 Notification Batching
 ~~~~~~~~~~~~~~~~~~~~~