Re: [RFC] fs: Avoid to use lockdep information if it's turned off

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Tue, Nov 10, 2020 at 01:40:16PM +0800, Boqun Feng wrote:
> On Mon, Nov 09, 2020 at 05:49:25PM -0800, Darrick J. Wong wrote:
> > On Tue, Nov 10, 2020 at 09:37:37AM +0800, Boqun Feng wrote:
> > > Filipe Manana reported a warning followed by task hanging after attempts
> > > to freeze a filesystem[1]. The problem happened in a LOCKDEP=y kernel,
> > > and percpu_rwsem_is_held() provided incorrect results when
> > > debug_locks == 0. Although the behavior is caused by commit 4d004099a668
> > > ("lockdep: Fix lockdep recursion"): after that lock_is_held() and its
> > > friends always return true if debug_locks == 0. However, one could argue
> > 
> > ...the silent trylock conversion with no checking of the return value is
> > completely broken.  I already sent a patch to tear all this out:
> > 
> > https://lore.kernel.org/linux-fsdevel/160494580419.772573.9286165021627298770.stgit@magnolia/T/#t
> > 
> 
> Thanks! That looks good to me. I'm all for removing that piece of code.
> 
> While we are at it, I have to ask, when you hit the original problem
> (warning after trylock in __start_sb_write()), did you see any lockdep
> splat happened previously?

Yes.  Every time I hit this there had been a lockdep splat earlier in the
fstests run, along with lockdep declaring that it was going offline.

--D

> Or just like Filipe, you hit that without
> seeing any lockdep splat happened before? Thanks! I'm trying to track
> down the silent lockdep turn-off.
> 
> Regards,
> Boqun
> 
> > --D
> > 
> > > that querying the lock holding information regardless if the lockdep
> > > turn-off status is inappropriate in the first place. Therefore instead
> > > of reverting lock_is_held() and its friends to the previous semantics,
> > > add the explicit checking in fs code to avoid use the lock holding
> > > information if lockdpe is turned off. And since the original problem
> > > also happened with a silent lockdep turn-off, put a warning if
> > > debug_locks is 0, which will help us spot the silent lockdep turn-offs.
> > > 
> > > [1]: https://lore.kernel.org/lkml/a5cf643b-842f-7a60-73c7-85d738a9276f@suse.com/
> > > 
> > > Reported-by: Filipe Manana <fdmanana@gmail.com>
> > > Fixes: 4d004099a668 ("lockdep: Fix lockdep recursion")
> > > Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > Cc: Jan Kara <jack@suse.cz>
> > > Cc: David Sterba <dsterba@suse.com>
> > > Cc: Nikolay Borisov <nborisov@suse.com>
> > > Cc: "Darrick J. Wong" <darrick.wong@oracle.com>
> > > ---
> > > Hi Filipe,
> > > 
> > > I use the slightly different approach to fix this problem, and I think
> > > it should have the similar effect with my previous fix[2], except that
> > > you will hit a warning if the problem happens now. The warning is added
> > > on purpose because I don't want to miss a silent lockdep turn-off.
> > > 
> > > Could you and other fs folks give this a try?
> > > 
> > > Regards,
> > > Boqun
> > > 
> > > [2]: https://lore.kernel.org/lkml/20201103140828.GA2713762@boqun-archlinux/
> > > 
> > >  fs/super.c | 11 +++++++++++
> > >  1 file changed, 11 insertions(+)
> > > 
> > > diff --git a/fs/super.c b/fs/super.c
> > > index a51c2083cd6b..1803c8d999e9 100644
> > > --- a/fs/super.c
> > > +++ b/fs/super.c
> > > @@ -1659,12 +1659,23 @@ int __sb_start_write(struct super_block *sb, int level, bool wait)
> > >  	 * twice in some cases, which is OK only because we already hold a
> > >  	 * freeze protection also on higher level. Due to these cases we have
> > >  	 * to use wait == F (trylock mode) which must not fail.
> > > +	 *
> > > +	 * Note: lockdep can only prove correct information if debug_locks != 0
> > >  	 */
> > >  	if (wait) {
> > >  		int i;
> > >  
> > >  		for (i = 0; i < level - 1; i++)
> > >  			if (percpu_rwsem_is_held(sb->s_writers.rw_sem + i)) {
> > > +				/*
> > > +				 * XXX: the WARN_ON_ONCE() here is to help
> > > +				 * track down silent lockdep turn-off, i.e.
> > > +				 * this warning is triggered, but no lockdep
> > > +				 * splat is reported.
> > > +				 */
> > > +				if (WARN_ON_ONCE(!debug_locks))
> > > +					break;
> > > +
> > >  				force_trylock = true;
> > >  				break;
> > >  			}
> > > -- 
> > > 2.29.2
> > >