On 16.01.20 16:54, Michal Hocko 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? I was asking myself for a long time: did anybody actually see this deadlock in real life? -- Thanks, David / dhildenb
