Re: [PATCH] lockdep: Optimize the memory usage of circular queue

[Dmitry Vyukov]

On Thu, Sep 24, 2020 at 5:13 PM Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
>
> Ping ;-)
>
> Regards,
> Boqun

Hi Boqun,

Peter says this may also fix this issue:
https://syzkaller.appspot.com/bug?extid=62ebe501c1ce9a91f68c
please add the following tag to the patch so that the bug report will
be closed on merge:
Reported-by: syzbot+62ebe501c1ce9a91f68c@xxxxxxxxxxxxxxxxxxxxxxxxx

> On Thu, Sep 17, 2020 at 04:01:50PM +0800, Boqun Feng wrote:
> > Qian Cai reported a BFS_EQUEUEFULL warning [1] after read recursive
> > deadlock detection merged into tip tree recently. Unlike the previous
> > lockep graph searching, which iterate every lock class (every node in
> > the graph) exactly once, the graph searching for read recurisve deadlock
> > detection needs to iterate every lock dependency (every edge in the
> > graph) once, as a result, the maximum memory cost of the circular queue
> > changes from O(V), where V is the number of lock classes (nodes or
> > vertices) in the graph, to O(E), where E is the number of lock
> > dependencies (edges), because every lock class or dependency gets
> > enqueued once in the BFS. Therefore we hit the BFS_EQUEUEFULL case.
> >
> > However, actually we don't need to enqueue all dependencies for the BFS,
> > because every time we enqueue a dependency, we almostly enqueue all
> > other dependencies in the same dependency list ("almostly" is because
> > we currently check before enqueue, so if a dependency doesn't pass the
> > check stage we won't enqueue it, however, we can always do in reverse
> > ordering), based on this, we can only enqueue the first dependency from
> > a dependency list and every time we want to fetch a new dependency to
> > work, we can either:
> >
> > 1)    fetch the dependency next to the current dependency in the
> >       dependency list
> > or
> > 2)    if the dependency in 1) doesn't exist, fetch the dependency from
> >       the queue.
> >
> > With this approach, the "max bfs queue depth" for a x86_64_defconfig +
> > lockdep and selftest config kernel can get descreased from:
> >
> >         max bfs queue depth:                   201
> >
> > to (after apply this patch)
> >
> >         max bfs queue depth:                   61
> >
> > While I'm at it, clean up the code logic a little (e.g. directly return
> > other than set a "ret" value and goto the "exit" label).
> >
> > [1]: https://lore.kernel.org/lkml/17343f6f7f2438fc376125384133c5ba70c2a681.camel@xxxxxxxxxx/
> >
> > Reported-by: Qian Cai <cai@xxxxxxxxxx>
> > Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> > ---
> >  kernel/locking/lockdep.c | 108 ++++++++++++++++++++++++---------------
> >  1 file changed, 67 insertions(+), 41 deletions(-)
> >
> > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > index cccf4bc759c6..761c2327e9cf 100644
> > --- a/kernel/locking/lockdep.c
> > +++ b/kernel/locking/lockdep.c
> > @@ -1640,35 +1640,22 @@ static enum bfs_result __bfs(struct lock_list *source_entry,
> >                            int offset)
> >  {
> >       struct lock_list *entry;
> > -     struct lock_list *lock;
> > +     struct lock_list *lock = NULL;
> >       struct list_head *head;
> >       struct circular_queue *cq = &lock_cq;
> > -     enum bfs_result ret = BFS_RNOMATCH;
> >
> >       lockdep_assert_locked();
> >
> > -     if (match(source_entry, data)) {
> > -             *target_entry = source_entry;
> > -             ret = BFS_RMATCH;
> > -             goto exit;
> > -     }
> > -
> > -     head = get_dep_list(source_entry, offset);
> > -     if (list_empty(head))
> > -             goto exit;
> > -
> >       __cq_init(cq);
> >       __cq_enqueue(cq, source_entry);
> >
> > -     while ((lock = __cq_dequeue(cq))) {
> > -             bool prev_only_xr;
> > -
> > -             if (!lock->class) {
> > -                     ret = BFS_EINVALIDNODE;
> > -                     goto exit;
> > -             }
> > +     while (lock || (lock = __cq_dequeue(cq))) {
> > +             if (!lock->class)
> > +                     return BFS_EINVALIDNODE;
> >
> >               /*
> > +              * Step 1: check whether we already finish on this one.
> > +              *
> >                * If we have visited all the dependencies from this @lock to
> >                * others (iow, if we have visited all lock_list entries in
> >                * @lock->class->locks_{after,before}) we skip, otherwise go
> > @@ -1676,17 +1663,17 @@ static enum bfs_result __bfs(struct lock_list *source_entry,
> >                * list accessed.
> >                */
> >               if (lock_accessed(lock))
> > -                     continue;
> > +                     goto next;
> >               else
> >                       mark_lock_accessed(lock);
> >
> > -             head = get_dep_list(lock, offset);
> > -
> > -             prev_only_xr = lock->only_xr;
> > -
> > -             list_for_each_entry_rcu(entry, head, entry) {
> > -                     unsigned int cq_depth;
> > -                     u8 dep = entry->dep;
> > +             /*
> > +              * Step 2: check whether prev dependency and this form a strong
> > +              *         dependency path.
> > +              */
> > +             if (lock->parent) { /* Parent exists, check prev dependency */
> > +                     u8 dep = lock->dep;
> > +                     bool prev_only_xr = lock->parent->only_xr;
> >
> >                       /*
> >                        * Mask out all -(S*)-> if we only have *R in previous
> > @@ -1698,29 +1685,68 @@ static enum bfs_result __bfs(struct lock_list *source_entry,
> >
> >                       /* If nothing left, we skip */
> >                       if (!dep)
> > -                             continue;
> > +                             goto next;
> >
> >                       /* If there are only -(*R)-> left, set that for the next step */
> > -                     entry->only_xr = !(dep & (DEP_SN_MASK | DEP_EN_MASK));
> > +                     lock->only_xr = !(dep & (DEP_SN_MASK | DEP_EN_MASK));
> > +             }
> >
> > -                     visit_lock_entry(entry, lock);
> > -                     if (match(entry, data)) {
> > -                             *target_entry = entry;
> > -                             ret = BFS_RMATCH;
> > -                             goto exit;
> > -                     }
> > +             /*
> > +              * Step 3: we haven't visited this and there is a strong
> > +              *         dependency path to this, so check with @match.
> > +              */
> > +             if (match(lock, data)) {
> > +                     *target_entry = lock;
> > +                     return BFS_RMATCH;
> > +             }
> > +
> > +             /*
> > +              * Step 4: if not match, expand the path by adding the
> > +              *         afterwards or backwards dependencis in the search
> > +              *
> > +              * Note we only enqueue the first of the list into the queue,
> > +              * because we can always find a sibling dependency from one
> > +              * (see label 'next'), as a result the space of queue is saved.
> > +              */
> > +             head = get_dep_list(lock, offset);
> > +             entry = list_first_or_null_rcu(head, struct lock_list, entry);
> > +             if (entry) {
> > +                     unsigned int cq_depth;
> > +
> > +                     if (__cq_enqueue(cq, entry))
> > +                             return BFS_EQUEUEFULL;
> >
> > -                     if (__cq_enqueue(cq, entry)) {
> > -                             ret = BFS_EQUEUEFULL;
> > -                             goto exit;
> > -                     }
> >                       cq_depth = __cq_get_elem_count(cq);
> >                       if (max_bfs_queue_depth < cq_depth)
> >                               max_bfs_queue_depth = cq_depth;
> >               }
> > +
> > +             /*
> > +              * Update the ->parent, so when @entry is iterated, we know the
> > +              * previous dependency.
> > +              */
> > +             list_for_each_entry_rcu(entry, head, entry)
> > +                     visit_lock_entry(entry, lock);
> > +next:
> > +             /*
> > +              * Step 5: fetch the next dependency to process.
> > +              *
> > +              * If there is a previous dependency, we fetch the sibling
> > +              * dependency in the dep list of previous dependency.
> > +              *
> > +              * Otherwise set @lock to NULL to fetch the next entry from
> > +              * queue.
> > +              */
> > +             if (lock->parent) {
> > +                     head = get_dep_list(lock->parent, offset);
> > +                     lock = list_next_or_null_rcu(head, &lock->entry,
> > +                                                  struct lock_list, entry);
> > +             } else {
> > +                     lock = NULL;
> > +             }
> >       }
> > -exit:
> > -     return ret;
> > +
> > +     return BFS_RNOMATCH;
> >  }
> >
> >  static inline enum bfs_result
> > --
> > 2.28.0
> >