On Tue, Jul 02, 2019 at 11:06:08AM +0200, Peter Zijlstra wrote: > On Mon, Jul 01, 2019 at 05:14:12PM -0700, Andrew Morton wrote: > > On Mon, 01 Jul 2019 01:27:04 -0700 syzbot > > <[email protected]> wrote: > > > > > Hello, > > > > > > syzbot found the following crash on: > > > > At a guess I'd say that perf_mmap() hit a deadlock on event->mmap_mutex > > while holding down_write(mmap_sem) (via vm_mmap_pgoff). The > > down_read(mmap_sem) in do_exit() happened to stumble across this and > > that's what got reported. > > lockdep never reported that and I don't see event->mmap_mutex being held > anywhere. > > AFAICT CPU0 is running 8355 and only 'has' mmap_sem -- it's blocked > waiting to acquire. > > CPU1 is running 8354 and has mmap_sem and is waiting to acquire > event->mmap_mutex. > > But nobody is actually owning it > > We take mmap_mutex in: > > perf_mmap() - called with mmap_sem held > perf_mmap_close() - called with mmap_sem held > > _free_event() - no faults/mmap while holding it > perf_poll() - idem > perf_event_set_output() - idem > > I don't see any of those functions in the below stacktrace, and having > just looked them over, I don't see how they would end up trying to > acquire mmap_sem and AB-BA. > > Now, clearly there's something screwy, but I'm not seeing a deadlock. > Let me go play with that reproducer. > > > > HEAD commit: 249155c2 Merge branch 'parisc-5.2-4' of > > > git://git.kernel.o.. > > > git tree: upstream > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1306be61a00000 > > > kernel config: https://syzkaller.appspot.com/x/.config?x=9a31528e58cc12e2 > > > dashboard link: > > > https://syzkaller.appspot.com/bug?extid=8cc1843d4eec9c0dfb35 > > > compiler: clang version 9.0.0 (/home/glider/llvm/clang
Also note that I very much do not trust that compiler to build a working kernel. There's still known code-gen bugs with it.
