New TTY layer: condvar(9) and Giant

[Attilio Rao]

2008/3/13, Ed Schouten <ed at 80386.nl>:
> Hello everyone,
>
>  Almost a month ago I started working on my assignment for my internship,
>  to reimplement a new TTY layer that fixes a lot of architectural
>  problems. So far, things are going quite fast:
>
>  - I've already implemented a basic TTY layer, which has support for
>   canonical and non-canonical mode. It still misses important features
>   including flow control, but it seems to work quite good. Unlike the
>   old layer, it doesn't buffer data as much, which should hopefully mean
>   it's a bit faster.
>  - I'm using a new PTY driver called pts(4). It works quite good, but it
>   misses the compatibility bits, which we'll need to have to support
>   older FreeBSD or Linux binaries.
>  - Some of you may have read I'm working on syscons now. I've got syscons
>   working with the new TTY layer; I'm typing this message through
>   syscons. ;-)
>
>  A lot of drivers that are used by the old TTY layer aren't mpsafe yet.
>  Of course, I'm willing to fix this, but this cannot be done in the
>  nearby future. This is why the new TTY layer should still allow TTY's to
>  be run under Giant.
>
>  In my initial implementation, each TTY device had its own mutex. In
>  theory, this is great. The PTY driver already uses this and it works
>  fine. There will be a lot of drivers, however, that want to use a
>  per-class mutex to lock all related TTY devices down at once (i.e.
>  syscons, which allocates 16 virtual TTY's). This is why I introduced a
>  per-class lock. When set to Giant, all TTY instances will lock down the
>  Giant lock when entering the TTY layer.
>
>  Unfortunately, I discovered condvar(9) can't properly unlock/lock the
>  Giant, which causes the system to panic. The condvar routines already
>  call DROP_GIANT before unlocking the lock itself.

I don't think we should allow this.
Giant is alredy too hidden inside other locking primitives creating a
lot of mis-understanding, mis-conceptions and mis-assumptions.

Attilio


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