linux-kernel - Re: [PATCH 0/3] KVM: Make kvm

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Message-ID: <20130922095348.GJ25202@redhat.com>
Date:	Sun, 22 Sep 2013 12:53:48 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	kvm@...r.kernel.org, jan.kiszka@...mens.com
Subject: Re: [PATCH 0/3] KVM: Make kvm_lock non-raw

On Sun, Sep 22, 2013 at 10:53:14AM +0200, Paolo Bonzini wrote:
> Il 22/09/2013 09:42, Gleb Natapov ha scritto:
> > On Mon, Sep 16, 2013 at 04:06:10PM +0200, Paolo Bonzini wrote:
> >> Paul Gortmaker reported a BUG on preempt-rt kernels, due to taking the
> >> mmu_lock within the raw kvm_lock in mmu_shrink_scan.  He provided a
> >> patch that shrunk the kvm_lock critical section so that the mmu_lock
> >> critical section does not nest with it, but in the end there is no reason
> >> for the vm_list to be protected by a raw spinlock.  Only manipulations
> >> of kvm_usage_count and the consequent hardware_enable/disable operations
> >> are not preemptable.
> >>
> >> This small series thus splits the kvm_lock in the "raw" part and the
> >> "non-raw" part.
> >>
> >> Paul, could you please provide your Tested-by?
> >>
> > Reviewed-by: Gleb Natapov <gleb@...hat.com>
> > 
> > But why should it go to stable?
> 
> It is a regression from before the kvm_lock was made raw.  Secondarily,
It was made raw in 2.6.39 and commit message claims that it is done for
-rt sake, why regression was noticed only now?

> it takes a much longer time before a patch hits -rt trees (can even be
> as much as a year) and this patch does nothing on non-rt trees.  So
> without putting it into stable it would get no actual coverage.
> 
The change is not completely trivial, it splits lock. There is no
obvious problem of course, otherwise you wouldn't send it and I
would ack it :), but it does not mean that the chance for problem is
zero, so why risk stability of stable even a little bit if the patch
does not fix anything in stable?

I do not know how -rt development goes and how it affects decisions for
stable acceptance, why can't they carry the patch in their tree until
they move to 3.12?

--
			Gleb.
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