HEADS DOWN: MPSAFE TTY layer integrated

Ed Schouten ed at 80386.nl
Wed Aug 20 09:16:53 UTC 2008

Hello everyone,

I'm pleased to announce that the MPSAFE TTY layer has just been
integrated to our source tree. Below is the commit message:

----- Forwarded message from Ed Schouten <ed at FreeBSD.org> -----
> Date: Wed, 20 Aug 2008 08:31:58 +0000 (UTC)
> From: Ed Schouten <ed at FreeBSD.org>
> To: src-committers at FreeBSD.org, cvs-src at FreeBSD.org, cvs-all at FreeBSD.org
> Subject: cvs commit: src ObsoleteFiles.inc UPDATING src/bin/sh miscbltin.c
> 	src/etc login.conf src/etc/defaults devfs.rules
> 	src/lib/libc/stdlib Makefile.inc Symbol.map grantpt.3 ptsname.3
> 	ptsname.c src/lib/libc/sys Makefile.inc Symbol.map getrlimit.2 ...
> ed          2008-08-20 08:31:58 UTC
>   FreeBSD src repository
>   <snip>
>   Log:
>   SVN rev 181905 on 2008-08-20 08:31:58Z by ed
>   Integrate the new MPSAFE TTY layer to the FreeBSD operating system.
>   The last half year I've been working on a replacement TTY layer for the
>   FreeBSD kernel. The new TTY layer was designed to improve the following:
>   - Improved driver model:
>     The old TTY layer has a driver model that is not abstract enough to
>     make it friendly to use. A good example is the output path, where the
>     device drivers directly access the output buffers. This means that an
>     in-kernel PPP implementation must always convert network buffers into
>     TTY buffers.
>     If a PPP implementation would be built on top of the new TTY layer
>     (still needs a hooks layer, though), it would allow the PPP
>     implementation to directly hand the data to the TTY driver.
>   - Improved hotplugging:
>     With the old TTY layer, it isn't entirely safe to destroy TTY's from
>     the system. This implementation has a two-step destructing design,
>     where the driver first abandons the TTY. After all threads have left
>     the TTY, the TTY layer calls a routine in the driver, which can be
>     used to free resources (unit numbers, etc).
>     The pts(4) driver also implements this feature, which means
>     posix_openpt() will now return PTY's that are created on the fly.
>   - Improved performance:
>     One of the major improvements is the per-TTY mutex, which is expected
>     to improve scalability when compared to the old Giant locking.
>     Another change is the unbuffered copying to userspace, which is both
>     used on TTY device nodes and PTY masters.
>   Upgrading should be quite straightforward. Unlike previous versions,
>   existing kernel configuration files do not need to be changed, except
>   when they reference device drivers that are listed in UPDATING.
>   Obtained from:          //depot/projects/mpsafetty/...
>   Approved by:            philip (ex-mentor)
>   Discussed:              on the lists, at BSDCan, at the DevSummit
>   Sponsored by:           Snow B.V., the Netherlands
>   dcons(4) fixed by:      kan
>   <snip>
----- End forwarded message -----

Some people asked me if I could tag the tree before importing the MPSAFE
TTY bits, to make sure people can easily stick to older source revisions
when they want to do comparisons with the old TTY implementation, which
may be useful when porting drivers, isolating bugs, etc.

After discussing with people on IRC, I've decided not to create the tag.
If you want to switch to pre-MPSAFE TTY, please check out revision

Well, that's all I've got to say for now, I guess. Be sure to update
your systems and give my code some extensive testing. Thanks!

 Ed Schouten <ed at 80386.nl>
 WWW: http://80386.nl/
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