[Qemu-devel] Re: Extending virtio_console to support multiple ports

[Amit Shah]

On (Thu) Aug 27 2009 [14:07:03], Benjamin Herrenschmidt wrote:
> On Wed, 2009-08-26 at 21:15 +0530, Amit Shah wrote:
> 
> > 
> > > - Convert hvc's usage of spinlocks to mutexes. I've no idea how this
> > >   will play out; I'm no expert here. But I did try doing this and so far
> > >   it all looks OK. No lockups, lockdep warnings, nothing. I have full
> > >   debugging enabled. But this doesn't mean it's right.
> > 
> > So just to test this further I added the capability to have more than
> > one hvc console spawn from virtio_console, created two consoles and did
> > a 'cat' of a file in each of the virtio-consoles. It's been running for
> > half an hour now without any badness. No spew in debug logs too.
> > 
> > I also checked the code in hvc_console.c that takes the spin_locks.
> > Nothing there that runs from (or needs to run from) interrupt context.
> > So the change to mutexes does seem reasonable. Also, the spinlock code
> > was added really long back -- git blame shows Linus' first git commit
> > introduced them in the git history, so it's pure legacy baggage.
> 
> Two things here:
> 
>  - First you seem to have completely missed the fact that hvc_poll() can
> be called from interrupt time :-) Look at hvc_irq.c which is used by

Right! That's the obvious one.

> some backends. Maybe that can be "fixed" by deferring to a work queue,
> though it's nice to have the keyboard input have somewhat of a higher
> priority than anything else here.

Hm, to maintain the current behaviour of poll() returning some
poll_mask, the poll_mask could be made into an atomic variable with
khvcd() updating it. But to have read at a higher priority than the
other stuff, I don't quite see yet how that can be done.

> So unless that's fixed, or I missed something, that's a big NACK for
> now.
> 
>  - Then, are we certain that there's no case where the tty layer will
> call us with some lock held or in an atomic context ? To be honest, I've

The other routines are open(), close(), write(), etc., and other kernel
context (hvc_instantiate() and the khvcd thread).

> totally lost track of the locking rules in tty land lately so it might
> well be ok, but something to verify.

Yes.

Thanks for the response!

		Amit