Messages in this thread | | | Date | Tue, 20 Oct 2015 08:42:19 -0400 | From | Jerome Glisse <> | Subject | Re: [PATCH] locking/lockdep: Fix expected depth value in __lock_release() |
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On Tue, Oct 20, 2015 at 02:18:53PM +0200, Peter Zijlstra wrote: > On Mon, Oct 19, 2015 at 03:30:28PM -0400, j.glisse@gmail.com wrote: > > From: Jérôme Glisse <jglisse@redhat.com> > > > > In __lock_release() we are removing one entry from the stack and > > rebuilding the hash chain by re-adding entry above the entry we > > just removed. If the entry removed was between 2 entry of same > > class then this 2 entry might be coalesced into one single entry > > which in turns means that the lockdep_depth value will not be > > incremented and thus the expected lockdep_depth value after this > > operation will be wrong triggering an unjustified WARN_ONCE() at > > the end of __lock_release(). > > This is the nest_lock stuff, right? Where it checks: > > if (hlock->class_idx == class_idx && nest_lock) { > ... > return 1; > }
Yes this code.
> > What code did you find that triggered this? That is, what code is taking > nested locks with other locks in the middle? (Not wrong per-se, just > curious how that would come about).
Well i am not able to reproduce myself but it happens as part of mm_drop_all_locks() as to which lock does trigger i am unsure as all the i_mmap_rwsem are taken one after the other and same for anon_vma rwsem so they should already coalesce properly. My guess is that code calling all lock also have a mutex and once all vma lock are drop the mutex coalesce with mm_all_locks_mutex.
> > > This patch adjust the expect depth value by decrementing it if > > what was previously 2 entry inside the stack are coalesced into > > only one entry. > > Would it not make more sense to scan the entire hlock stack on > __lock_acquire() and avoid getting collapsible entries in the first > place? > > Something like so...
It would work too, probably more compute intensive than my solution but this is lockdep code so i guess it is fine. Also dunno if we loose any valuable information by not keeping the stack ordered so one can check order in whick lock are taken.
> > --- > kernel/locking/lockdep.c | 22 ++++++++++++++-------- > 1 file changed, 14 insertions(+), 8 deletions(-) > > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c > index 4e49cc4c9952..6fcd98b86e7b 100644 > --- a/kernel/locking/lockdep.c > +++ b/kernel/locking/lockdep.c > @@ -3125,15 +3125,21 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass, > > class_idx = class - lock_classes + 1; > > - if (depth) { > - hlock = curr->held_locks + depth - 1; > - if (hlock->class_idx == class_idx && nest_lock) { > - if (hlock->references) > - hlock->references++; > - else > - hlock->references = 2; > + if (depth && nest_lock) { > + int i; > > - return 1; > + for (i = depth; i; --i) { > + hlock = curr->held_locks + i - 1; > + if (hlock->class_idx == class_idx && > + hlock->nest_lock == nest_lock) { > + > + if (hlock->references) > + hlock->references++; > + else > + hlock->references = 2; > + > + return 1; > + } > } > } >
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