Re: lockdep WARNING in get_online_cpus

[Dmitry Vyukov]

On Mon, Apr 4, 2016 at 11:01 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Mon, Apr 04, 2016 at 10:19:05AM +0200, Dmitry Vyukov wrote:
>>
>> This happens in CONFIG_DEBUG_LOCKDEP code. Is it a bug in lockdep?
>
> We hope not; but it is a new test.
>
> So lockdep needs to check each current lock stack against the recorded
> lock dependencies to see if we've gotten ourselves a cycle. Doing this
> check is _expensive_.
>
> So what lockdep does is it computes a hash for each lock stack and only
> if we've not seen this hash before (actually truncated since we don't
> have a full 64bit hashtable) do we go look for cycles.
>
> The new check tries to detect hash-collisions in this cache. A collision
> would result in not checking for cycles, even if we've not seen the
> stack before.
>
> You've managed to tickle this.
>
> Now, last week I found some bugs in there, and Alfredo added a pretty
> printer, so maybe try and add these patches to your testing?
>
>   git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git locking/urgent
>
>> But I always see at the same stack involving perf and jump_label...
>
> So you have a simple reproducer? So that I can have a go at this.


Yes, I am able to reproduce it by running:

# ./syz-execprog  -cover=0 -repeat=0 -procs=20 -nobody=0
crash-qemu-26-1459761514194788294

crash-qemu-26-1459761514194788294 being:
https://gist.githubusercontent.com/dvyukov/bce719c2b7dede54f96d8bbb7c78714f/raw/08a7a237986f0e3bccc7c0e800a3bdaa32e1fc0b/gistfile1.txt