Re: soft lockup in kvm_flush_remote_tlbs

Avi Kivity
2007-10-25T13:46:11Z

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.

--
error compiling committee.c: too many arguments to function