Re: soft lockup in kvm_flush_remote_tlbs

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

The issue appears to be with the RHEL5 kernel (host OS is rhel5). I tried your suggestions below -- no effect; still hit the softlockup.

I then moved the host to the 2.6.23.1 kernel but with the kvm-48 code base. Surprisingly, I had no issues starting my guest with '-smp 4'.

david


Avi Kivity wrote:  
> 
> 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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