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

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Message-Id: <20230111183031.2449668-1-pbonzini@redhat.com>
Date:   Wed, 11 Jan 2023 13:30:31 -0500
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     dwmw@...zon.co.uk, seanjc@...gle.com
Subject: [PATCH] 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 <pbonzini@...hat.com>
---
 Documentation/virt/kvm/locking.rst | 23 ++++++++++++-----------
 1 file changed, 12 insertions(+), 11 deletions(-)

diff --git a/Documentation/virt/kvm/locking.rst b/Documentation/virt/kvm/locking.rst
index 897ca39b72bf..53826098183e 100644
--- a/Documentation/virt/kvm/locking.rst
+++ b/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:
 
-- 
2.39.0