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

[Benjamin Herrenschmidt]

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
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.

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

Cheers,
Ben.