linux-kernel - Re: [PATCH 1/2] KVM: x86: fix usage of kvm_lock in set_nx_huge

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Message-ID: <Z5Qz3OGxuRH_vj_G@google.com>
Date: Fri, 24 Jan 2025 16:44:12 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH 1/2] KVM: x86: fix usage of kvm_lock in set_nx_huge_pages()

On Sat, Jan 25, 2025, Paolo Bonzini wrote:
> On 1/25/25 00:44, Sean Christopherson wrote:
> > SRCU readers would only interact with kvm_destroy_vm() from a locking perspective,
> > and if that's problematic then we would already have a plethora of issues.
> 
> Ah yeah, I missed that you cannot hold any lock when calling kvm_put_kvm().
> So the waiting side is indeed a leaf and cannot block someone else.
> 
> Still from your patch (thanks!) I don't really like the special cases on
> taking SRCU vs. kvm_lock... It really seems like a job for a mutex or rwsem.
> It keeps the complexity in the one place that is different (i.e. where a
> lock is taken inside the iteration) and everything else can just iterate
> normally.

I like the special casing, it makes the oddballs stand out, which in turn (hopefully)
makes developers pause and take note.  I.e. the SRCU walkers are all normal readers,
the set_nx_huge_pages() "never" path is a write in disguise, and
kvm_hyperv_tsc_notifier() is a very special snowflake.