Re: [PATCH RFC 02/10] KVM: Allow reading memslots while holding slots_arch_lock

[Marco Elver]

On Thu, 10 Sept 2026 at 18:30, Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Thu, Sep 10, 2026, Marco Elver wrote:
> > kvm_swap_active_memslots() updates kvm->memslots[as_id] while holding both
> > kvm->slots_lock and kvm->slots_arch_lock. Holding either lock guarantees
> > that memslots cannot be concurrently modified.
>
> Sure, but that's irrelevant.  The goal of the srcu_dereference_check() is to
> ensure that readers see a stable view of the VM's overall memory, not simply that
> kvm->memslots can't be written.

Functionally, this is irrelevant for readers. But under lockdep it
isn't for writers: srcu_dereference_check() (with lockdep) asserts
that the srcu reader-lock is held, or the condition 'c' holds, which
here is holding any of the writer locks.

> > Allow reading memslots in __kvm_memslots() when kvm->slots_arch_lock is
> > held.
>
> Why?

Holding any of the writer locks guarantees no concurrent modification;
therefore, if any writer lock is held, it's not required that the srcu
reader-lock is held. There are few places where only either slots_lock
or slots_arch_lock is held, which is sufficient for reading.