linux-kernel - Re: [PATCH v3 1/3] KVM: fix rcu usage warning in kvm_arch_vcpu_ioctl_set

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Message-ID: <20110120152736.GA3922@amt.cnet>
Date:	Thu, 20 Jan 2011 13:27:36 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
Cc:	Avi Kivity <avi@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
	KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v3 1/3] KVM: fix rcu usage warning in
 kvm_arch_vcpu_ioctl_set_sregs()

On Thu, Jan 20, 2011 at 11:03:31AM +0800, Xiao Guangrong wrote:
> On 01/20/2011 02:13 AM, Marcelo Tosatti wrote:
> > On Wed, Jan 19, 2011 at 01:16:23PM +0800, Xiao Guangrong wrote:
> >> On 01/12/2011 03:39 PM, Xiao Guangrong wrote:
> >>> Fix:
> >>>
> >>> [ 1001.499596] ===================================================
> >>> [ 1001.499599] [ INFO: suspicious rcu_dereference_check() usage. ]
> >>> [ 1001.499601] ---------------------------------------------------
> >>> [ 1001.499604] include/linux/kvm_host.h:301 invoked rcu_dereference_check() without protection!
> >>> 	......
> >>> [ 1001.499636] Pid: 6035, comm: qemu-system-x86 Not tainted 2.6.37-rc6+ #62
> >>> [ 1001.499638] Call Trace:
> >>> [ 1001.499644]  [] lockdep_rcu_dereference+0x9d/0xa5
> >>> [ 1001.499653]  [] gfn_to_memslot+0x8d/0xc8 [kvm]
> >>> [ 1001.499661]  [] gfn_to_hva+0x16/0x3f [kvm]
> >>> [ 1001.499669]  [] kvm_read_guest_page+0x1e/0x5e [kvm]
> >>> [ 1001.499681]  [] kvm_read_guest_page_mmu+0x53/0x5e [kvm]
> >>> [ 1001.499699]  [] load_pdptrs+0x3f/0x9c [kvm]
> >>> [ 1001.499705]  [] ? vmx_set_cr0+0x507/0x517 [kvm_intel]
> >>> [ 1001.499717]  [] kvm_arch_vcpu_ioctl_set_sregs+0x1f3/0x3c0 [kvm]
> >>> [ 1001.499727]  [] kvm_vcpu_ioctl+0x6a5/0xbc5 [kvm]
> >>>
> >>> Signed-off-by: Xiao Guangrong <xiaoguangrong@...fujitsu.com>
> >>
> >> Ping ...?
> > 
> > Applied this fix. For the make_all_cpus_request optimization, can you
> > show numbers with this new version? Because now there is LOCK# similarly
> > to the spinlock.
> 
> Marcelo,
> 
> Sure :-), there is the simply test result of kernbench:
> 
> Before patch:
> 
> real    5m6.493s                                                                
> user    3m57.847s                                                               
> sys     9m7.115s 
> 
> real    5m1.750s                                                                
> user    4m0.109s                                                                
> sys     9m10.192s       
> 
> 
> After patch:
> real    5m0.140s                                                                
> user    3m57.956s                                                               
> sys     8m58.339s  
> 
> real    4m56.314s                                                               
> user    4m0.303s                                                                
> sys     8m55.774s  

Nice. One disadvantageous side effect for the kvm_vcpu_kick path 
is that it can race with make_all_cpus_request, which is possibly
doing unrelated, slower work (IPI'ing other vcpus, waiting for
response).

Looks ok, but lets wait for more careful reviews before applying.


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