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SubjectRe: [RESEND PATCH 01/11] kernel:lockdep:print the shortest dependency chain if finding a circle
On Sun, Jun 28, 2009 at 11:04:36PM +0800, tom.leiming@gmail.com wrote:
> From: Ming Lei <tom.leiming@gmail.com>
>
> Currently lockdep will print the 1st circle detected if it exists when
> acquiring a new (next) lock.
>
> This patch prints the shortest path from the next lock to be acquired to
> the previous held lock if a circle is found.
>
> The patch still uses the current method to check circle, and once the
> circle is found, breadth-first search algorithem is used to compute the
> shortest path from the next lock to the previous lock in the forward
> lock dependency graph.
>
> Printing the shortest path will shorten the dependency chain, and make
> troubleshooting for possible circular locking easier.



Oh! That looks fairly different from what I was thinking about...

If I understand well, lets imagine the following:

Task 1 acquires: A B F
Task 2 acquires: A B C D E F
Task 3 acquires: F B

Before your patch, DFS would report the BF - FB wicked dependency
by reporting the Task 2 dependency snapshot, which is cluttered by
the other locks C, D and E (that would be reported in the dependency chain
if I'm not wrong) whereas BFS would be smarter by finding the shortest
snapshot found in Task 3: just F B.

Correct me if I misunderstand this patch.

I suggest you to provide an example along this patch, that would make
it easier to demonstrate its importance.

Thanks,
Frederic.



> Signed-off-by: Ming Lei <tom.leiming@gmail.com>
> ---
> include/linux/lockdep.h | 6 ++
> kernel/lockdep.c | 115 ++++++++++++++++++++++++++++++++++++++++----
> kernel/lockdep_internals.h | 83 +++++++++++++++++++++++++++++++
> 3 files changed, 195 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
> index b25d1b5..9ec026f 100644
> --- a/include/linux/lockdep.h
> +++ b/include/linux/lockdep.h
> @@ -149,6 +149,12 @@ struct lock_list {
> struct lock_class *class;
> struct stack_trace trace;
> int distance;
> +
> + /*The parent field is used to implement breadth-first search,and
> + *the bit 0 is reused to indicate if the lock has been accessed
> + *in BFS.
> + */
> + struct lock_list *parent;
> };
>
> /*
> diff --git a/kernel/lockdep.c b/kernel/lockdep.c
> index 8bbeef9..93dc70d 100644
> --- a/kernel/lockdep.c
> +++ b/kernel/lockdep.c
> @@ -897,6 +897,79 @@ static int add_lock_to_list(struct lock_class *class, struct lock_class *this,
> return 1;
> }
>
> +static struct circular_queue lock_cq;
> +static int __search_shortest_path(struct lock_list *source_entry,
> + struct lock_class *target,
> + struct lock_list **target_entry,
> + int forward)
> +{
> + struct lock_list *entry;
> + struct circular_queue *cq = &lock_cq;
> + int ret = 1;
> +
> + __cq_init(cq);
> +
> + mark_lock_accessed(source_entry, NULL);
> + if (source_entry->class == target) {
> + *target_entry = source_entry;
> + ret = 0;
> + goto exit;
> + }
> +
> + __cq_enqueue(cq, (unsigned long)source_entry);
> +
> + while (!__cq_empty(cq)) {
> + struct lock_list *lock;
> + struct list_head *head;
> +
> + __cq_dequeue(cq, (unsigned long *)&lock);
> +
> + if (!lock->class) {
> + ret = -2;
> + goto exit;
> + }
> +
> + if (forward)
> + head = &lock->class->locks_after;
> + else
> + head = &lock->class->locks_before;
> +
> + list_for_each_entry(entry, head, entry) {
> + if (!lock_accessed(entry)) {
> + mark_lock_accessed(entry, lock);
> + if (entry->class == target) {
> + *target_entry = entry;
> + ret = 0;
> + goto exit;
> + }
> +
> + if (__cq_enqueue(cq, (unsigned long)entry)) {
> + ret = -1;
> + goto exit;
> + }
> + }
> + }
> + }
> +exit:
> + return ret;
> +}
> +
> +static inline int __search_forward_shortest_path(struct lock_list *src_entry,
> + struct lock_class *target,
> + struct lock_list **target_entry)
> +{
> + return __search_shortest_path(src_entry, target, target_entry, 1);
> +
> +}
> +
> +static inline int __search_backward_shortest_path(struct lock_list *src_entry,
> + struct lock_class *target,
> + struct lock_list **target_entry)
> +{
> + return __search_shortest_path(src_entry, target, target_entry, 0);
> +
> +}
> +
> /*
> * Recursive, forwards-direction lock-dependency checking, used for
> * both noncyclic checking and for hardirq-unsafe/softirq-unsafe
> @@ -934,7 +1007,7 @@ print_circular_bug_header(struct lock_list *entry, unsigned int depth)
> {
> struct task_struct *curr = current;
>
> - if (!debug_locks_off_graph_unlock() || debug_locks_silent)
> + if (debug_locks_silent)
> return 0;
>
> printk("\n=======================================================\n");
> @@ -954,19 +1027,41 @@ print_circular_bug_header(struct lock_list *entry, unsigned int depth)
> return 0;
> }
>
> -static noinline int print_circular_bug_tail(void)
> +static noinline int print_circular_bug(void)
> {
> struct task_struct *curr = current;
> struct lock_list this;
> + struct lock_list *target;
> + struct lock_list *parent;
> + int result;
> + unsigned long depth;
>
> - if (debug_locks_silent)
> + if (!debug_locks_off_graph_unlock() || debug_locks_silent)
> return 0;
>
> this.class = hlock_class(check_source);
> if (!save_trace(&this.trace))
> return 0;
>
> - print_circular_bug_entry(&this, 0);
> + result = __search_forward_shortest_path(&this,
> + hlock_class(check_target),
> + &target);
> + if (result) {
> + printk("\n%s:search shortest path failed:%d\n", __func__,
> + result);
> + return 0;
> + }
> +
> + depth = get_lock_depth(target);
> +
> + print_circular_bug_header(target, depth);
> +
> + parent = get_lock_parent(target);
> +
> + while (parent) {
> + print_circular_bug_entry(parent, --depth);
> + parent = get_lock_parent(parent);
> + }
>
> printk("\nother info that might help us debug this:\n\n");
> lockdep_print_held_locks(curr);
> @@ -1072,14 +1167,15 @@ check_noncircular(struct lock_class *source, unsigned int depth)
> */
> list_for_each_entry(entry, &source->locks_after, entry) {
> if (entry->class == hlock_class(check_target))
> - return print_circular_bug_header(entry, depth+1);
> + return 2;
> debug_atomic_inc(&nr_cyclic_checks);
> - if (!check_noncircular(entry->class, depth+1))
> - return print_circular_bug_entry(entry, depth+1);
> + if (check_noncircular(entry->class, depth+1) == 2)
> + return 2;
> }
> return 1;
> }
>
> +
> #if defined(CONFIG_TRACE_IRQFLAGS) && defined(CONFIG_PROVE_LOCKING)
> /*
> * Forwards and backwards subgraph searching, for the purposes of
> @@ -1484,8 +1580,9 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
> */
> check_source = next;
> check_target = prev;
> - if (!(check_noncircular(hlock_class(next), 0)))
> - return print_circular_bug_tail();
> + if (check_noncircular(hlock_class(next), 0) == 2)
> + return print_circular_bug();
> +
>
> if (!check_prev_add_irq(curr, prev, next))
> return 0;
> diff --git a/kernel/lockdep_internals.h b/kernel/lockdep_internals.h
> index 699a2ac..6f48d37 100644
> --- a/kernel/lockdep_internals.h
> +++ b/kernel/lockdep_internals.h
> @@ -136,3 +136,86 @@ extern atomic_t nr_find_usage_backwards_recursions;
> # define debug_atomic_dec(ptr) do { } while (0)
> # define debug_atomic_read(ptr) 0
> #endif
> +
> +/* The circular_queue and helpers is used to implement the
> + * breadth-first search(BFS)algorithem, by which we can build
> + * the shortest path from the next lock to be acquired to the
> + * previous held lock if there is a circular between them.
> + * */
> +#define MAX_CIRCULAR_QUE_SIZE 4096UL
> +struct circular_queue{
> + unsigned long element[MAX_CIRCULAR_QUE_SIZE];
> + unsigned int front, rear;
> +};
> +
> +#define LOCK_ACCESSED 1UL
> +#define LOCK_ACCESSED_MASK (~LOCK_ACCESSED)
> +
> +static inline void __cq_init(struct circular_queue *cq)
> +{
> + cq->front = cq->rear = 0;
> +}
> +
> +static inline int __cq_empty(struct circular_queue *cq)
> +{
> + return (cq->front == cq->rear);
> +}
> +
> +static inline int __cq_full(struct circular_queue *cq)
> +{
> + return ((cq->rear + 1)%MAX_CIRCULAR_QUE_SIZE) == cq->front;
> +}
> +
> +static inline int __cq_enqueue(struct circular_queue *cq, unsigned long elem)
> +{
> + if (__cq_full(cq))
> + return -1;
> +
> + cq->element[cq->rear] = elem;
> + cq->rear = (cq->rear + 1)%MAX_CIRCULAR_QUE_SIZE;
> + return 0;
> +}
> +
> +static inline int __cq_dequeue(struct circular_queue *cq, unsigned long *elem)
> +{
> + if (__cq_empty(cq))
> + return -1;
> +
> + *elem = cq->element[cq->front];
> + cq->front = (cq->front + 1)%MAX_CIRCULAR_QUE_SIZE;
> + return 0;
> +}
> +
> +static inline int __cq_get_elem_count(struct circular_queue *cq)
> +{
> + return (cq->rear - cq->front)%MAX_CIRCULAR_QUE_SIZE;
> +}
> +
> +static inline void mark_lock_accessed(struct lock_list *lock,
> + struct lock_list *parent)
> +{
> + lock->parent = (void *) parent + LOCK_ACCESSED;
> +}
> +
> +static inline unsigned long lock_accessed(struct lock_list *lock)
> +{
> + return (unsigned long)lock->parent & LOCK_ACCESSED;
> +}
> +
> +static inline struct lock_list *get_lock_parent(struct lock_list *child)
> +{
> + return (struct lock_list *)
> + ((unsigned long)child->parent & LOCK_ACCESSED_MASK);
> +}
> +
> +static inline unsigned long get_lock_depth(struct lock_list *child)
> +{
> + unsigned long depth = 0;
> + struct lock_list *parent;
> +
> + while ((parent = get_lock_parent(child))) {
> + child = parent;
> + depth++;
> + }
> + return depth;
> +}
> --
> 1.6.0.GIT
>
> --
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