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Subject[PATCH] Documentation: kvm: clarify SRCU locking order
Date
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: Paolo Bonzini <pbonzini@redhat.com>
---
Documentation/virt/kvm/locking.rst | 19 ++++++++++++++-----
1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/Documentation/virt/kvm/locking.rst b/Documentation/virt/kvm/locking.rst
index 845a561629f1..a3ca76f9be75 100644
--- a/Documentation/virt/kvm/locking.rst
+++ b/Documentation/virt/kvm/locking.rst
@@ -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
--
2.31.1
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