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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
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