linux-kernel - Re: [PATCH] KVM: x86/mmu: Ensure NX huge page recovery thread is alive before waking

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Message-ID: <Z5fO5bac8ohqUH1D@kbusch-mbp>
Date: Mon, 27 Jan 2025 11:22:29 -0700
From: Keith Busch <kbusch@...nel.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: Ensure NX huge page recovery thread is
 alive before waking

On Mon, Jan 27, 2025 at 08:48:03AM -0800, Sean Christopherson wrote:
> > > -		vhost_task_start(kvm->arch.nx_huge_page_recovery_thread);
> > > +	if (!nx_thread)
> > > +		return;
> > > +
> > > +	vhost_task_start(nx_thread);
> > > +
> > > +	/* Make the task visible only once it is fully started. */
> > > +	WRITE_ONCE(kvm->arch.nx_huge_page_recovery_thread, nx_thread);
> > 
> > I believe the WRITE_ONCE needs to happen before the vhost_task_start to
> > ensure the parameter update callback can see it before it's started.
> 
> It's not clear to me that calling vhost_task_wake() before vhost_task_start() is
> allowed, which is why I deliberately waited until the task was started to make it
> visible.  Though FWIW, doing "vhost_task_wake(nx_thread)" before vhost_task_start()
> doesn't explode.

Hm, it does look questionable to try to wake a process that hadn't been
started yet, but I think it may be okay: task state will be TASK_NEW
before vhost_task_start(), which looks like will cause wake_up_process()
to do nothing.