Re: [PATCH 1/1] KVM: Fix potentially recursively get kvm lock

[Gleb Natapov <gleb@redhat.com>]

On Tue, May 12, 2009 at 11:30:21AM -0300, Marcelo Tosatti wrote:
> On Tue, May 12, 2009 at 10:13:36PM +0800, Yang, Sheng wrote:
> > > > +	mutex_unlock(&kvm->lock);
> > >
> > > assigned_dev list is protected by kvm->lock. So you could have another
> > > ioctl adding to it at the same time you're searching.
> > 
> > Oh, yes... My fault... 
> > 
> > > Could either have a separate kvm->assigned_devs_lock, to protect
> > > kvm->arch.assigned_dev_head (users are ioctls that manipulate it), or
> > > change the IRQ injection to use a separate spinlock, kill the workqueue
> > > and call kvm_set_irq from the assigned device interrupt handler.
> > 
> > Peferred the latter, though needs more work. But the only reason for put a 
> > workqueue here is because kvm->lock is a mutex? I can't believe... If so, I 
> > think we had made a big mistake - we have to fix all kinds of racy problem 
> > caused by this, but finally find it's unnecessary... 
> 
> One issue is that kvm_set_irq can take too long while interrupts are
> blocked, and you'd have to disable interrupts in other contexes that
> inject interrupts (say qemu->ioctl(SET_INTERRUPT)->...->), so all i can
> see is a tradeoff.
> 
> <guess mode on>
> 
> But the interrupt injection path seems to be pretty short and efficient
> to happen in host interrupt context.
> 
> <guess mode off>
> 
> Avi, Gleb?
> 
Interrupt injection path also use IRQ routing data structures so access
to them should be protected by the same lock. And of cause in kernel
device (apic/ioapic/pic) mmio is done holding this lock so interrupt
injection cannot happen in parallel with device reconfiguration. May
be we want more parallelism here.

> > Maybe another reason is kvm_kick_vcpu(), but have already fixed by you.
> 
> Note you tested the spinlock_irq patch with GigE and there was no
> significant performance regression right?
> 
> > 
> > Continue to check the code...
> > 
> > -- 
> > regards
> > Yang, Sheng
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--
			Gleb.