Commit 3a9ae31a authored by Paolo Bonzini's avatar Paolo Bonzini
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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: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 45e966fc

Documentation/virt/kvm/locking.rst

+12 −11
Original line number Diff line number Diff line
@@ -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: