Re: soft lockup in kvm_flush_remote_tlbs

[david ahern <daahern-FYB4Gu1CFyUAvxtiuMwx3w@public.gmane.org>]

I'll give your suggestions I try. I need to move to a server that I can forcibly reboot remotely (to recover), so it will be a while.

david


Avi Kivity wrote:
> david ahern wrote:
>> As a quick test I added a printk to the loop, right after the while():
>>
>>      while (atomic_read(&completed) != needed) {
>>          printk("kvm_flush_remote_tlbs: completed = %d, needed = %d\n", atomic_read(&completed), needed);
>>          cpu_relax();
>>          barrier();
>>      }
>>
>>
>> This is the output right before a lockup:
>>
>> Oct 24 16:03:47 bldr-ccm20 kernel: kvm_flush_remote_tlbs: completed = 2, needed = 2
>> Oct 24 16:03:47 bldr-ccm20 kernel: kvm_flush_remote_tlbs: completed = 2, needed = 2
>> Oct 24 16:03:47 bldr-ccm20 kernel: kvm_flush_remote_tlbs: completed = 1, needed = 2
>> Oct 24 16:03:57 bldr-ccm20 last message repeated 105738 times
>> Oct 24 16:03:57 bldr-ccm20 kernel: BUG: soft lockup detected on CPU#0!
>> Oct 24 16:03:57 bldr-ccm20 kernel:  [<c044a0b7>] softlockup_tick+0x98/0xa6
>> Oct 24 16:03:57 bldr-ccm20 kernel:  [<c042cc98>] update_process_times+0x39/0x5c
>> Oct 24 16:03:57 bldr-ccm20 kernel:  [<c04176ec>] smp_apic_timer_interrupt+0x5c/0x64
>> Oct 24 16:03:57 bldr-ccm20 kernel:  [<c04049bf>] apic_timer_interrupt+0x1f/0x24
>> Oct 24 16:03:57 bldr-ccm20 kernel:  [<c0424130>] vprintk+0x288/0x2bc
>> Oct 24 16:03:57 bldr-ccm20 kernel:  [<c0459db7>] follow_page+0x168/0x1b6
>> Oct 24 16:03:57 bldr-ccm20 kernel:  [<c04d8067>] cfq_slice_async_store+0x5/0x38
>> Oct 24 16:03:57 bldr-ccm20 kernel:  [<c0459db7>] follow_page+0x168/0x1b6
>> Oct 24 16:03:57 bldr-ccm20 kernel:  [<c0406406>] do_IRQ+0xa5/0xae
>> Oct 24 16:03:57 bldr-ccm20 kernel:  [<c040492e>] common_interrupt+0x1a/0x20
>> Oct 24 16:03:57 bldr-ccm20 kernel:  [<c042417c>] printk+0x18/0x8e
>> Oct 24 16:03:57 bldr-ccm20 kernel:  [<f89a9812>] kvm_flush_remote_tlbs+0xe0/0xf2 [kvm]
>> ...
>>
>>
>> I'd like to get a solution for RHEL5, so I am attempting to backport smp_call_function_mask(). I'm open to other suggestions if you think it is corruption or the problem is somewhere else.
>>   
> 
> No, it looks like the problem is indeed in kvm_flush_remote_tlbs(), and 
> not a corruption elsewhere.
> 
> Things to check:
> 
> - whether cpus_weight(mask) == needed
> - whether wrapping the whole thing in preempt_disable()/preempt_enable() 
> helps
> 
> hey! I see a bug!
> 
>>                         continue;
>>                 cpu = vcpu->cpu;
>>                 if (cpu != -1 && cpu != raw_smp_processor_id())
>>                         if (!cpu_isset(cpu, cpus)) {
>>                                 cpu_set(cpu, cpus);
>>                                 ++needed;
>>                         }
>>         }
>>
> 
> vcpu->cpu can change during execution if this snippet due to a vcpu 
> being migrated concurrently with this being executed.  Since the 
> compiler is free to reload 'cpu' from 'vcpu->cpu', the code can operate 
> on corrupted data.
> 
> A 'barrier();' after 'cpu = vcpu->cpu;' should fix it, if this is indeed 
> the bug.
> 
> 

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