Documentation: kvm: fix SRCU locking order docs

kvm->srcu is taken in KVM_RUN and several other vCPU ioctls, therefore
vcpu->mutex is susceptible to the same deadlock that is documented
for kvm->slots_lock.  The same holds for kvm->lock, since kvm->lock
is held outside vcpu->mutex.  Fix the documentation and rearrange it
to highlight the difference between these locks and kvm->slots_arch_lock,
and how kvm->slots_arch_lock can be useful while processing a vmexit.

Signed-off-by: Paolo Bonzini <[email protected]>

Documentation/virt/kvm/locking.rst

@@ -24,17 +24,18 @@ The acquisition orders for mutexes are as follows:
 
 For SRCU:
 
-- ``synchronize_srcu(&kvm->srcu)`` is called _inside_
-  the kvm->slots_lock critical section, therefore kvm->slots_lock
-  cannot be taken inside a kvm->srcu read-side critical section.
-  Instead, kvm->slots_arch_lock is released before the call
-  to ``synchronize_srcu()`` and _can_ be taken inside a
-  kvm->srcu read-side critical section.
-
-- kvm->lock is taken inside kvm->srcu, therefore
-  ``synchronize_srcu(&kvm->srcu)`` cannot be called inside
-  a kvm->lock critical section.  If you cannot delay the
-  call until after kvm->lock is released, use ``call_srcu``.
+- ``synchronize_srcu(&kvm->srcu)`` is called inside critical sections
+  for kvm->lock, vcpu->mutex and kvm->slots_lock.  These locks _cannot_
+  be taken inside a kvm->srcu read-side critical section; that is, the
+  following is broken::
+
+      srcu_read_lock(&kvm->srcu);
+      mutex_lock(&kvm->slots_lock);
+
+- kvm->slots_arch_lock instead is released before the call to
+  ``synchronize_srcu()``.  It _can_ therefore be taken inside a
+  kvm->srcu read-side critical section, for example while processing
+  a vmexit.
 
 On x86: