From: Corinna Vinschen <corinna-cygwin@cygwin.com>
To: cygwin-developers@cygwin.com
Subject: Re: The unreliability of AF_UNIX datagram sockets
Date: Tue, 4 May 2021 13:06:43 +0200 [thread overview]
Message-ID: <YJEqw5+qxCzCG79S@calimero.vinschen.de> (raw)
In-Reply-To: <1aa91ec3-ae10-0e83-470d-a8a2dcfd83b0@cornell.edu>
On May 3 16:50, Ken Brown wrote:
> On 5/3/2021 3:48 PM, Ken Brown wrote:
> > On 5/3/2021 2:40 PM, Corinna Vinschen wrote:
> > > On May 3 12:56, Ken Brown wrote:
> > > > On 5/3/2021 11:45 AM, Corinna Vinschen wrote:
> > > > > 7. The idea of _mq_recv partial reads is entirely broken. Given that
> > > > > the information in the queue consists of header info plus payload,
> > > > > the entire block has to be read, and then a new block with fixed
> > > > > header and shortened payload has to be rewritten with bumped priority.
> > > > > This in turn can only be performed by the AF_UNIX code, unless we
> > > > > expect knowledge of the AF_UNIX packet layout in the mqueue code.
> > > >
> > > > The partial read is actually OK as is, since it's comparable to what happens
> > > > on a partial read from a pipe. I already have AF_UNIX code (on the
> > > > topic/af_unix branch) that deals with that. A boolean variable _unread
> > > > keeps track of whether there's unread data from a previous partial read. If
> > > > so, the next read just reads data without expecting a header.
> > >
> > > Ok, never mind.
> > >
> > > One advantage of the mqueue when utilized as above would be that this
> > > kind of state info is not required. The content of a packet would
> > > always be self-contained and bumping the priority would automagically
> > > move the packet content to the top of the queue. But that's just
> > > idle musing at this point.
> >
> > I thought about that but rejected it for the following reason: Suppose
> > the receiver reads a message and tries to rewrite it with modified
> > header, shortened payload, and bumped priority. The sender might have
> > already written more messages between the read and the write, and the
> > queue could be full.
> >
> > Now that I'm rethinking this, however, maybe we could get around that
> > problem with an internal _mq_lock function that would block senders
> > while the receiver decides whether it needs to do a partial read.
>
> Alternatively, _mq_recv could accept an _MQ_LOCK flag, which means "don't
> release the mutex", and then there could be an _mq_unlock function, which
> simply releases the mutex.
That's an idea. However, I think this is something we can push back for
now, and ultimately we can use any of the above solutions which makes most
sense. Implemanting a defered unlock if required is not much of a problem.
Corinna
next prev parent reply other threads:[~2021-05-04 11:06 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-27 15:47 Ken Brown
2021-04-29 11:05 ` Corinna Vinschen
2021-04-29 11:16 ` Corinna Vinschen
2021-04-29 14:38 ` Ken Brown
2021-04-29 15:05 ` Corinna Vinschen
2021-04-29 15:18 ` Corinna Vinschen
2021-04-29 16:44 ` Ken Brown
2021-04-29 17:39 ` Corinna Vinschen
2021-05-01 21:41 ` Ken Brown
2021-05-03 10:30 ` Corinna Vinschen
2021-05-03 15:45 ` Corinna Vinschen
2021-05-03 16:56 ` Ken Brown
2021-05-03 18:40 ` Corinna Vinschen
2021-05-03 19:48 ` Ken Brown
2021-05-03 20:50 ` Ken Brown
2021-05-04 11:06 ` Corinna Vinschen [this message]
2021-05-13 14:30 ` Ken Brown
2021-05-17 10:26 ` Corinna Vinschen
2021-05-17 13:02 ` Ken Brown
2021-05-17 13:02 ` Ken Brown
2021-05-20 13:46 ` Ken Brown
2021-05-20 19:25 ` Corinna Vinschen
2021-05-21 21:54 ` Ken Brown
2021-05-22 15:49 ` Corinna Vinschen
2021-05-22 16:50 ` Ken Brown
2021-05-22 18:21 ` Ken Brown
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=YJEqw5+qxCzCG79S@calimero.vinschen.de \
--to=corinna-cygwin@cygwin.com \
--cc=cygwin-developers@cygwin.com \
/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
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
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 read-only IMAP folder(s) and NNTP newsgroup(s).