linux-kernel - [PATCH] Documentation: kvm: clarify SRCU locking order

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Message-Id: <20221228110410.1682852-2-pbonzini@redhat.com>
Date:   Wed, 28 Dec 2022 06:04:10 -0500
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     seanjc@...gle.com
Subject: [PATCH] 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: Paolo Bonzini <pbonzini@...hat.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