Re: mmu_notifiers: turn off lockdep around mm_take_all_locks

[Avi Kivity <avi@redhat.com>]

On 07/07/2009 09:15 PM, Peter Zijlstra wrote:
> On Tue, 2009-07-07 at 15:06 -0300, Marcelo Tosatti wrote:
>    
>> KVM guests with CONFIG_LOCKDEP=y trigger the following warning:
>>
>> BUG: MAX_LOCK_DEPTH too low!
>> turning off the locking correctness validator.
>> Pid: 4624, comm: qemu-system-x86 Not tainted 2.6.31-rc2-03981-g3abaf21
>> #32
>> Call Trace:
>>   [<ffffffff81068bab>] __lock_acquire+0x1559/0x15fc
>>   [<ffffffff810be4d9>] ? mm_take_all_locks+0x99/0x109
>>   [<ffffffff810be4d9>] ? mm_take_all_locks+0x99/0x109
>>   [<ffffffff81068d3c>] lock_acquire+0xee/0x112
>>   [<ffffffff810be516>] ? mm_take_all_locks+0xd6/0x109
>>   [<ffffffff81402596>] ? _spin_lock_nest_lock+0x20/0x50
>>   [<ffffffff814025b7>] _spin_lock_nest_lock+0x41/0x50
>>   [<ffffffff810be516>] ? mm_take_all_locks+0xd6/0x109
>>   [<ffffffff810be516>] mm_take_all_locks+0xd6/0x109
>>   [<ffffffff810d0f76>] do_mmu_notifier_register+0xd4/0x199
>>   [<ffffffff810d1060>] mmu_notifier_register+0x13/0x15
>>   [<ffffffffa0107f16>] kvm_dev_ioctl+0x13f/0x30e [kvm]
>>   [<ffffffff810e6a3a>] vfs_ioctl+0x2f/0x7d
>>   [<ffffffff810e6fb7>] do_vfs_ioctl+0x4af/0x4ec
>>   [<ffffffff814030b4>] ? error_exit+0x94/0xb0
>>   [<ffffffff81401f92>] ? trace_hardirqs_off_thunk+0x3a/0x3c
>>   [<ffffffff8100bc2d>] ? retint_swapgs+0xe/0x13
>>   [<ffffffff810e703b>] sys_ioctl+0x47/0x6a
>>   [<ffffffff811d849c>] ? __up_read+0x1a/0x85
>>   [<ffffffff8100b1db>] system_call_fastpath+0x16/0x1b
>>
>> Since mm_take_all_locks takes a gazillion locks.
>>
>> Is there any way around this other than completly shutting down lockdep?
>>      
>
> When we created this the promise was that kvm would only do this on a
> fresh mm with only a few vmas, has that changed

The number of vmas did increase, but not materially.  We do link with 
more shared libraries though.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.