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

[Darrick J. Wong]

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

--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@xxxxxxxx/
> 
> Reported-by: Filipe Manana <fdmanana@xxxxxxxxx>
> Fixes: 4d004099a668 ("lockdep: Fix lockdep recursion")
> Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: David Sterba <dsterba@xxxxxxxx>
> Cc: Nikolay Borisov <nborisov@xxxxxxxx>
> Cc: "Darrick J. Wong" <darrick.wong@xxxxxxxxxx>
> ---
> 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
>