On Thu 16-01-20 11:05:07, Qian Cai wrote: > > > > On Jan 16, 2020, at 10:54 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > > > On Thu 16-01-20 09:53:13, Qian Cai wrote: > >> > >> > >>> On Jan 16, 2020, at 9:28 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote: > >>> > >>> On Wed 15-01-20 12:29:16, Qian Cai wrote: > >>>> It is guaranteed to trigger a lockdep splat if calling printk() with > >>>> zone->lock held because there are many places (tty, console drivers, > >>>> debugobjects etc) would allocate some memory with another lock > >>>> held which is proved to be difficult to fix them all. > >>> > >>> I am still not happy with the above much. What would say about something > >>> like below instead? > >>> " > >>> It is not that hard to trigger lockdep splats by calling printk from > >>> under zone->lock. Most of them are false positives caused by lock chains > >>> introduced early in the boot process and they do not cause any real > >>> problems. There are some console drivers which do allocate from the > >>> printk context as well and those should be fixed. In any case false > >>> positives are not that trivial to workaround and it is far from optimal > >>> to lose lockdep functionality for something that is a non-issue. > >>> <An example of such a false positive goes here> > >>> " > >> > >> I feel like I repeated myself too many times. A call trace for one lock dependency > >> is sometimes from early boot process because lockdep will save the first one it > >> encountered, but it does not mean the lock dependency will only not happen in > >> early boot. I spent some time to study those early boot call traces in the given > >> lockdep splats, and it looks to me the lock dependency is also possible after > >> the boot. > > > > Then state it explicitly with an example of the trace and explanation > > that the deadlock is real. If the deadlock is real then it shouldn't be > > really terribly hard to notice even without lockdep splats which get > > disabled after the first false positive, right? > > A deadlock could be really hard to trigger though which needs a perfect > timing between multiple threads. All I am saying is: Do not speculate in changelog. Make clear arguments. So far we have seen many false positives and that is stated in the wording I have suggested. It is also explained why those suck. There is also a note that _some_ consoles might indeed deadlock. Compare that to the original changelog which doesn't really saying anything useful about those lockdep splats. I obviously do not insist on my wording but please make the changelog clear on the actual problem and stick to facts. Thanks! -- Michal Hocko SUSE Labs
