Commit 02d9a04d authored by Paolo Bonzini's avatar Paolo Bonzini
Browse files

Documentation: kvm: clarify SRCU locking order 



Currently only the locking order of SRCU vs kvm->slots_arch_lock
and kvm->slots_lock is documented.  Extend this to kvm->lock
since Xen emulation got it terribly wrong.

Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent a79b53aa

Documentation/virt/kvm/locking.rst

+14 −5
Original line number Diff line number Diff line
@@ -16,17 +16,26 @@ The acquisition orders for mutexes are as follows: 
- kvm->slots_lock is taken outside kvm->irq_lock, though acquiring

  them together is quite rare.



- Unlike kvm->slots_lock, kvm->slots_arch_lock is released before

  synchronize_srcu(&kvm->srcu).  Therefore kvm->slots_arch_lock

  can be taken inside a kvm->srcu read-side critical section,

  while kvm->slots_lock cannot.



- kvm->mn_active_invalidate_count ensures that pairs of

  invalidate_range_start() and invalidate_range_end() callbacks

  use the same memslots array.  kvm->slots_lock and kvm->slots_arch_lock

  are taken on the waiting side in install_new_memslots, so MMU notifiers

  must not take either kvm->slots_lock or kvm->slots_arch_lock.



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``.



On x86:



- vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock