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.
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?
