LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Dominique Martinet <asmadeus@codewreck.org>
To: Tom Herbert <tom@quantonium.net>
Cc: Tom Herbert <tom@herbertland.com>,
	David Miller <davem@davemloft.net>,
	Doron Roberts-Kedes <doronrk@fb.com>,
	Dave Watson <davejwatson@fb.com>,
	Linux Kernel Network Developers <netdev@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] kcm: remove any offset before parsing messages
Date: Fri, 15 Feb 2019 05:52:14 +0100	[thread overview]
Message-ID: <20190215045214.GA13123@nautica> (raw)
In-Reply-To: <CAPDqMeoJ7CCo1eGNBp_-crkxfVt_4f=XQqhEo7kmyCN-hf_EWQ@mail.gmail.com>

Tom Herbert wrote on Thu, Feb 14, 2019:
> On Thu, Feb 14, 2019 at 7:31 PM Dominique Martinet
> <asmadeus@codewreck.org> wrote:
> > Yes, the parser fails with -ENOMEM ; that is not handled gracefully at
> > all: from a user point of view, the connection just hangs (recvmsg never
> > returns), without so much as a message in dmesg either.
>
> That's by design. Here is the description in kcm.txt:
> 
> "When a TCP socket is attached to a KCM multiplexor data ready
> (POLLIN) and write space available (POLLOUT) events are handled by the
> multiplexor. If there is a state change (disconnection) or other error
> on a TCP socket, an error is posted on the TCP socket so that a
> POLLERR event happens and KCM discontinues using the socket. When the
> application gets the error notification for a TCP socket, it should
> unattach the socket from KCM and then handle the error condition (the
> typical response is to close the socket and create a new connection if
> necessary)."

Sigh, that's what I get for relying on pieces of code found on the
internet.

It does make "trivial" kcm sockets difficult to use though, the old
ganesha code I adapted to kcm was the horrible (naive?) kind spawning
one thread per connection and just waiting on read(), so making it just
create a kcm socket from the tcp one and wait on recvmsg() until an
error pops up does not seem an option.

That being said I'm a bit confused, I thought part of the point of kcm
was the multiplexing so a simple server could just keep attaching tcp
sockets to a single kcm socket and only have a single trivial loop
reading from the kcm socket ; but I guess there's no harm in also
looking for POLLERR on the tcp sockets... It would still need to close
them once clients disconnect I guess, this really only affects my naive
server.

> > strparser might be able to retry somehow.
> 
> We could retry on -ENOMEM since it is potentially a transient error,

Yes, I think we should aim to retry on -ENOMEM; I agree all errors are
not meant to be retried on but there already are different cases based
on what the parse cb returned; but that can be a different patch.

> but generally for errors (like connection is simply broken) it seems
> like it's working as intended. I suppose we could add a socket option
> to fail the KCM socket when all attached lower sockets have failed,
> but I not sure that would be a significant benefit for additional
> complexity.

Right, I agree it's probably not worth doing, now I'm aware of this I
can probably motivate myself to change this old code to use epoll
properly.

I think it could make sense to have this option for simpler programs,
but as things stand I guess we can require kcm users to do this much,
thanks for the explanation, and sorry for having failed to notice it.



With all that said I guess my patch should work correctly then, I'll try
to find some time to check the error does come back up the tcp socket in
my reproducer but I have no reason to believe it doesn't.

I'd like to see some retry on ENOMEM before this is merged though, so
while I'm there I'll resend this with a second patch doing that
retry,.. I think just not setting strp->interrupted and not reporting
the error up might be enough? Will have to try either way.



Thanks for the feedback,
-- 
Dominique

  reply	other threads:[~2019-02-15  4:52 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-09-11  9:21 Dominique Martinet
2018-09-12  5:36 ` Dominique Martinet
2018-09-18  1:45   ` David Miller
2018-09-18  1:57     ` Dominique Martinet
2018-09-18  2:40       ` David Miller
2018-09-18  2:45         ` Dominique Martinet
2018-09-18  2:51           ` David Miller
2018-09-18  2:58             ` Dominique Martinet
2018-10-31  2:56       ` Dominique Martinet
2019-02-15  1:00         ` Dominique Martinet
2019-02-15  1:20           ` Tom Herbert
2019-02-15  1:57             ` Dominique Martinet
2019-02-15  2:48               ` Tom Herbert
2019-02-15  3:31                 ` Dominique Martinet
2019-02-15  4:01                   ` Tom Herbert
2019-02-15  4:52                     ` Dominique Martinet [this message]
2019-02-20  4:11                       ` Dominique Martinet
2019-02-20 16:18                         ` Tom Herbert
2019-02-21  8:22                           ` Dominique Martinet
2019-02-22 19:24                             ` Tom Herbert
2019-02-22 20:27                               ` Dominique Martinet
2019-02-22 21:01                                 ` Tom Herbert

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20190215045214.GA13123@nautica \
    --to=asmadeus@codewreck.org \
    --cc=davejwatson@fb.com \
    --cc=davem@davemloft.net \
    --cc=doronrk@fb.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=netdev@vger.kernel.org \
    --cc=tom@herbertland.com \
    --cc=tom@quantonium.net \
    --subject='Re: [PATCH v2] kcm: remove any offset before parsing messages' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).