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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
