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-7a60-73c7-85d738a92...@suse.com/ > > Reported-by: Filipe Manana <fdman...@gmail.com> > Fixes: 4d004099a668 ("lockdep: Fix lockdep recursion") > Signed-off-by: Boqun Feng <boqun.f...@gmail.com> > Cc: Peter Zijlstra <pet...@infradead.org> > Cc: Jan Kara <j...@suse.cz> > Cc: David Sterba <dste...@suse.com> > Cc: Nikolay Borisov <nbori...@suse.com> > Cc: "Darrick J. Wong" <darrick.w...@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 >
