Messages in this thread | | | Date | Mon, 14 Jan 2019 14:36:50 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] locking/lockdep: Add debug_locks check in __lock_downgrade() |
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On Thu, Jan 10, 2019 at 11:21:13AM +0100, Dmitry Vyukov wrote: > On Thu, Jan 10, 2019 at 5:04 AM Waiman Long <longman@redhat.com> wrote: > > > > Tetsuo Handa had reported he saw an incorrect "downgrading a read lock" > > warning right after a previous lockdep warning. It is likely that the > > previous warning turned off lock debugging causing the lockdep to have > > inconsistency states leading to the lock downgrade warning. > > > > Fix that by add a check for debug_locks at the beginning of > > __lock_downgrade(). > > > > Signed-off-by: Waiman Long <longman@redhat.com> > > Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp> > > Please also add: > > Reported-by: syzbot+53383ae265fb161ef488@syzkaller.appspotmail.com > > for tracking purposes. But Tetsuo deserves lots of credit for debugging it.
I made that:
Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp> Debugged-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp> Reported-by: syzbot+53383ae265fb161ef488@syzkaller.appspotmail.com
> > index 9593233..e805fe3 100644 > > --- a/kernel/locking/lockdep.c > > +++ b/kernel/locking/lockdep.c > > @@ -3535,6 +3535,9 @@ static int __lock_downgrade(struct lockdep_map *lock, unsigned long ip) > > unsigned int depth; > > int i; > > > > + if (unlikely(!debug_locks)) > > + return 0; > > + > > Are we sure this resolves the problem rather than makes the > inconsistency window smaller? > I don't understand all surrounding code, but looking just at this > function it looks like it may just pepper over the problem. Say, we > pass this check when lockdep was still turned on. Then this thread is > preempted for some time (e.g. a virtual CPU), then another thread > started reporting a warning, turned lockdep off, some information > wasn't collected, and this this task resumes and reports a false > warning.
Theoretically possible I suppose; but this is analogous to many of the other lockdep hooks.
> Or we are holding the mutex here, and the fact that we are holding it > ensures that no other task will take it and no information will be > lost?
There is no lock here; for performance reasons we prefer not to acquire a global spinlock on every lockdep hook, that would be horrific.
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