Re: Review/Test: Pseudo-device unit number management patch

On Monday 09 February 2004 16:54, Poul-Henning Kamp wrote:
> In message <20040209150039.GS2803_at_pcwin002.win.tue.nl>, Stijn Hoop writes:
> >On Mon, Feb 09, 2004 at 03:32:16PM +0100, Poul-Henning Kamp wrote:
> >> Please also remember:  /dev is _not_ an inventory of available devices.
> >
> >Just curious: what is it then? An inventory of available drivers? An
> >inventory of open devices?
>
> 	"A naming-space gateway from filenames to device drivers
> 	whose exact mapping is only stable in the timeinterval
> 	between a successful open(2) and the corresponding close
> 	(be it an explict close or not)." [2]
>
> Fortunately it is a good deal more deterministic than what my feeble
> attempt at a definition above could make it sound :-)
>
> The practical effects are hard to explain, but one example is that
> the stat/open race is much more fundamental in /dev than anywhere
> else in the filesystem.
>
> In the normal filesystem, a few odd things may happen between calls
> to stat(2) and open(2), in /proc you may get a different process
> than you intended, but in /dev _any_ odd thing may happen.
>
> You thought you opened a serial port called /dev/foo connected to
> your printer ?  Well, I got news for you:  it disappeared!

How does this work, when there's no mechanism in place to signal "the return 
of the thingy". Mostly thinking about the keyboard here, because even if 
there was a recheck_keyboard_existence utility, one couldn't really type it 
in. So far I haven't been able to plug-in the keyboard and get it back again.

This is also a problem with some kvm-switches (the more expensive ones seem to 
be able to fool the OS, but the ones you buy just for changing co-locaters 
don't). I haven't checked with PC-Weazle cards (VGA/mouse/keyboard 
redirection to serial outlet for ia32/pci) and RELENG_5_x yet, but if these 
loose their keyboard as well, there's definitely a problem.
-- 
Melvyn

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FreeBSD sarevok.idg.nl 5.2-CURRENT FreeBSD 5.2-CURRENT #2: Mon Feb 16 19:59:52 
CET 2004     
root_at_sarevok.webteckies.org:/usr/obj/usr/src/sys/SAREVOK_NOAPM_NODEBUG  i386
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