kern_yield vs ukbd_yield

[Hans Petter Selasky]

On Sunday 18 December 2011 11:58:57 Andriy Gapon wrote:
> on 17/12/2011 19:06 Hans Petter Selasky said the following:
> > If the problem is only in UKBD driver, I don't think this is a big
> > problem to solve. The reason for the auto-magic locking, is that I've
> > sometimes seen callers in non-polling mode call into UKBD without Giant
> > locked. And this causes a panic when starting USB transfers, because the
> > code expects Giant to be locked.
> 
> Hmm, do you have an example of such a panic? I couldn't find how this could
> be possible in my examination of non-polling entry points into syscons and
> kbd drives.

mtx_assert(&Giant, MTX_OWNED); will panic if Giant is not locked. Do you need 
an example?

> 
> > Requirement: I would say that to use UKBD polling mode, the scheduler
> > must be stopped. Else you should not use polling mode at all. I think
> > that mountroot is getting characters the wrong way in this regard.
> 
> I think that this is a too strong / unnecessary requirement.  I think that
> now (after my recent syscons commits) ukbd should work correctly and
> optimally in this context.
> 
> I have written a small article about my analysis of locking in syscons,
> underlying kbd drivers and ukbd in particular:
> http://wiki.freebsd.org/AvgSyscons
> I could have misunderstood or missed things, so I will appreciate a review
> and discussion of my observations.  Thank you!

I will have a look at it when I find some time!

--HPS