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Date: Sat, 16 Nov 2013 17:55:17 +1100
From: Paul Mackerras <paulus@samba.org>
To: Liu Ping Fan <qemulist@gmail.com>
Subject: Re: [PATCH v4] powerpc: kvm: fix rare but potential deadlock scene
Message-ID: <20131116065517.GA18339@iris.ozlabs.ibm.com>
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Cc: linuxppc-dev@lists.ozlabs.org, Alexander Graf <agraf@suse.de>,
 kvm-ppc@vger.kernel.org
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On Fri, Nov 15, 2013 at 04:35:00PM +0800, Liu Ping Fan wrote:
> Since kvmppc_hv_find_lock_hpte() is called from both virtmode and
> realmode, so it can trigger the deadlock.
> 
> Suppose the following scene:
> 
> Two physical cpuM, cpuN, two VM instances A, B, each VM has a group of
> vcpus.
> 
> If on cpuM, vcpu_A_1 holds bitlock X (HPTE_V_HVLOCK), then is switched
> out, and on cpuN, vcpu_A_2 try to lock X in realmode, then cpuN will be
> caught in realmode for a long time.
> 
> What makes things even worse if the following happens,
>   On cpuM, bitlockX is hold, on cpuN, Y is hold.
>   vcpu_B_2 try to lock Y on cpuM in realmode
>   vcpu_A_2 try to lock X on cpuN in realmode
> 
> Oops! deadlock happens
> 
> Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>

Reviewed-by: Paul Mackerras <paulus@samba.org>