On Tue, Apr 26, 2011 at 10:53:04AM +0200, Gerd Hoffmann wrote: > Hi, > > [ ... back online now ... ] > > >>/var/tmp/portage/app-emulation/qemu-kvm-0.14.0/work/qemu-kvm-0.14.0/qemu-kvm.c:1724: > >>kvm_mutex_unlock: Assertion `!cpu_single_env' failed. > > >That's a spice bug. In fact, there are a lot of > >qemu_mutex_lock/unlock_iothread in that subsystem. I bet at least a few > >of them can cause even more subtle problems. > > > >Two general issues with dropping the global mutex like this: > > - The caller of mutex_unlock is responsible for maintaining > > cpu_single_env across the unlocked phase (that's related to the > > abort above). > > This is true for qemu-kvm only, right? > > qemu-kvm specific patches which add the cpu_single_env tracking (not > polished yet) are here: > > http://cgit.freedesktop.org/spice/qemu/log/?h=spice.kvm.v28 > > > - Dropping the lock in the middle of a callback is risky. That may > > enable re-entrances of code sections that weren't designed for this > > Hmm, indeed. > > >Spice requires a careful review regarding such issues. Or it should > >pioneer with introducing its own lock so that we can handle at least > >related I/O activities over the VCPUs without holding the global mutex > >(but I bet it's not the simplest candidate for such a new scheme). > > spice/qxl used to have its own locking scheme. That didn't work out > though. spice server is threaded and calls back into qxl from spice > thread context, and some of these callbacks need access to qemu data > structures (display surface) and thus lock protection which covers > more than just the spice subsystem. > > I'll look hard again whenever I can find a way out of this > (preferably drop the need for the global lock somehow). For now I'm > pretty busy with the email backlog though ... > We (Hans, Uri, and Me) have already sent a fix for this, it seems to have passed everyone's testing, and it basically does just that - drops the use of the mutex. It's in http://cgit.freedesktop.org/spice/qemu/log/?h=spice.v32.kvm, see the patches: qxl/spice-display: move pipe to ssd qxl: implement get_command in vga mode without locks qxl/spice: remove qemu_mutex_{un,}lock_iothread around dispatcher hw/qxl-render: drop cursor locks, replace with pipe And specifically the comments too. Alon > cheers, > Gerd > -- > 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 -- 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
