On Thu, Apr 28, 2022 at 5:59 PM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> On 4/28/22 23:28, Peter Gonda wrote:
> >
> > So when actually trying this out I noticed that we are releasing the
> > current vcpu iterator but really we haven't actually taken that lock
> > yet. So we'd need to maintain a prev_* pointer and release that one.
>
> Not entirely true because all vcpu->mutex.dep_maps will be for the same
> lock. The dep_map is essentially a fancy string, in this case
> "&vcpu->mutex".
>
> See the definition of mutex_init:
>
> #define mutex_init(mutex) \
> do { \
> static struct lock_class_key __key; \
> \
> __mutex_init((mutex), #mutex, &__key); \
> } while (0)
>
> and the dep_map field is initialized with
>
> lockdep_init_map_wait(&lock->dep_map, name, key, 0, LD_WAIT_SLEEP);
>
> (i.e. all vcpu->mutexes share the same name and key because they have a
> single mutex_init-ialization site). Lockdep is as crude in theory as it
> is effective in practice!
>
> >
> > bool acquired = false;
> > kvm_for_each_vcpu(...) {
> > if (!acquired) {
> > if (mutex_lock_killable_nested(&vcpu->mutex, role)
> > goto out_unlock;
> > acquired = true;
> > } else {
> > if (mutex_lock_killable(&vcpu->mutex, role)
> > goto out_unlock;
>
> This will cause a lockdep splat because it uses subclass 0. All the
> *_nested functions is allow you to specify a subclass other than zero.
OK got it. I now have this to lock:
kvm_for_each_vcpu (i, vcpu, kvm) {
if (prev_vcpu != NULL) {
mutex_release(&prev_vcpu->mutex.dep_map, _THIS_IP_);
prev_vcpu = NULL;
}
if (mutex_lock_killable_nested(&vcpu->mutex, role))
goto out_unlock;
prev_vcpu = vcpu;
}
But I've noticed the unlocking is in the wrong order since we are
using kvm_for_each_vcpu() I think we need a kvm_for_each_vcpu_rev() or
something. Which maybe a bit for work:
https://elixir.bootlin.com/linux/latest/source/lib/xarray.c#L1119.
Then I think we could something like this to unlock:
bool acquired = true;
kvm_for_each_vcpu_rev(i, vcpu, kvm) {
if (!acquired) {
mutex_acquire(&vcpu->mutex.dep_map, 0, role,
_THIS_IP_);
}
mutex_unlock(&vcpu->mutex);
acquired = false;
}
>
> Paolo
>
> > }
> > }
> >
> > To unlock:
> >
> > kvm_for_each_vcpu(...) {
> > mutex_unlock(&vcpu->mutex);
> > }
> >
> > This way instead of mocking and releasing the lock_dep we just lock
> > the fist vcpu with mutex_lock_killable_nested(). I think this
> > maintains the property you suggested of "coalesces all the mutexes for
> > a vm in a single subclass". Thoughts?
>
