Re: [PATCH 3.18 031/104] locking/lockdep: Add debug_locks check in __lock_downgrade()

Greg Kroah-Hartman
2019-04-25T07:56:48Z

On Thu, Apr 25, 2019 at 02:30:35AM +0900, Tetsuo Handa wrote:
> On 2019/04/25 2:08, Greg Kroah-Hartman wrote:
> > [ Upstream commit 71492580571467fb7177aade19c18ce7486267f5 ]
> >
> > 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().
>
> Excuse me?
>
> >
> > Debugged-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
> > Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
> > Reported-by: syzbot+53383ae265fb161ef488@syzkaller.appspotmail.com
> > Signed-off-by: Waiman Long <longman@redhat.com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Will Deacon <will.deacon@arm.com>
> > Link: https://lkml.kernel.org/r/1547093005-26085-1-git-send-email-longman@redhat.com
> > Signed-off-by: Ingo Molnar <mingo@kernel.org>
> > Signed-off-by: Sasha Levin <sashal@kernel.org>
> > ---
> > kernel/locking/lockdep.c | 3 +++
> > 1 file changed, 3 insertions(+)
> >
> > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > index fb90ca3a296e..27de98428367 100644
> > --- a/kernel/locking/lockdep.c
> > +++ b/kernel/locking/lockdep.c
> > @@ -3312,6 +3312,9 @@ __lock_set_class(struct lockdep_map *lock, const char *name,
> > unsigned int depth;
> > int i;
> >
> > + if (unlikely(!debug_locks))
> > + return 0;
> > +
>
> This is __lock_set_class() function rather than __lock_downgrade() function.
> __lock_downgrade() is available in 4.12+.
>
> For 3.18-stable, downgrade_write() is in kernel/locking/rwsem.c . Therefore,
> we should check whether adding this check into __lock_set_class() is what
> we want to do...

Ugh, this got backported incorrectly to 4.4.y as well, let me check
4.9.y... Yup, that's wrong too.

As is 4.14.y.

As is 4.19.y.

Hey, I got it right in 5.0.y!

Ok, let me go drop this 3.18.y patch, and revert it from all of the
other trees for now as well, thanks for catching this.

greg k-h