On Mon, Apr 4, 2016 at 11:01 AM, Peter Zijlstra <[email protected]> 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
