Re: [PATCH v4 05/15] lockdep: Make check_prev_add can use a separate stack_trace

Byungchul Park
2017-01-18T02:19:47Z

On Tue, Jan 17, 2017 at 04:54:31PM +0100, Peter Zijlstra wrote:
> On Fri, Jan 13, 2017 at 07:11:43PM +0900, Byungchul Park wrote:
> > What do you think about the following patches doing it?
>
> I was more thinking about something like so...
>
> Also, I think I want to muck with struct stack_trace; the members:
> max_nr_entries and skip are input arguments to save_stack_trace() and
> bloat the structure for no reason.

With your approach, save_trace() must be called whenever check_prevs_add()
is called, which might be unnecessary.

Frankly speaking, I think what I proposed resolved it neatly. Don't you
think so?

>
> ---
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 7c38f8f3d97b..f2df300a96ee 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -430,6 +430,21 @@ static int save_trace(struct stack_trace *trace)
> return 1;
> }
>
> +static bool return_trace(struct stack_trace *trace)
> +{
> + /*
> + * If @trace is the last trace generated by save_trace(), then we can
> + * return the entries by simply subtracting @nr_stack_trace_entries
> + * again.
> + */
> + if (trace->entries != stack_trace + nr_stack_trace_entries - trace->nr_entres)
> + return false;
> +
> + nr_stack_trace_entries -= trace->nr_entries;
> + trace->entries = NULL;
> + return true;
> +}
> +
> unsigned int nr_hardirq_chains;
> unsigned int nr_softirq_chains;
> unsigned int nr_process_chains;
> @@ -1797,20 +1812,12 @@ static inline void inc_chains(void)
> */
> static int
> check_prev_add(struct task_struct *curr, struct held_lock *prev,
> - struct held_lock *next, int distance, int *stack_saved)
> + struct held_lock *next, int distance, struct stack_trace *trace)
> {
> struct lock_list *entry;
> int ret;
> struct lock_list this;
> struct lock_list *uninitialized_var(target_entry);
> - /*
> - * Static variable, serialized by the graph_lock().
> - *
> - * We use this static variable to save the stack trace in case
> - * we call into this function multiple times due to encountering
> - * trylocks in the held lock stack.
> - */
> - static struct stack_trace trace;
>
> /*
> * Prove that the new <prev> -> <next> dependency would not
> @@ -1858,11 +1865,7 @@ static inline void inc_chains(void)
> }
> }
>
> - if (!*stack_saved) {
> - if (!save_trace(&trace))
> - return 0;
> - *stack_saved = 1;
> - }
> + trace->skip = 1; /* mark used */
>
> /*
> * Ok, all validations passed, add the new lock
> @@ -1870,14 +1873,14 @@ static inline void inc_chains(void)
> */
> ret = add_lock_to_list(hlock_class(next),
> &hlock_class(prev)->locks_after,
> - next->acquire_ip, distance, &trace);
> + next->acquire_ip, distance, trace);
>
> if (!ret)
> return 0;
>
> ret = add_lock_to_list(hlock_class(prev),
> &hlock_class(next)->locks_before,
> - next->acquire_ip, distance, &trace);
> + next->acquire_ip, distance, trace);
> if (!ret)
> return 0;
>
> @@ -1885,8 +1888,6 @@ static inline void inc_chains(void)
> * Debugging printouts:
> */
> if (verbose(hlock_class(prev)) || verbose(hlock_class(next))) {
> - /* We drop graph lock, so another thread can overwrite trace. */
> - *stack_saved = 0;
> graph_unlock();
> printk("\n new dependency: ");
> print_lock_name(hlock_class(prev));
> @@ -1908,10 +1909,15 @@ static inline void inc_chains(void)
> static int
> check_prevs_add(struct task_struct *curr, struct held_lock *next)
> {
> + struct stack_trace trace = { .nr_entries = 0, .skip = 0, };
> int depth = curr->lockdep_depth;
> - int stack_saved = 0;
> struct held_lock *hlock;
>
> + if (!save_trace(&trace))
> + goto out_bug;
> +
> + trace.skip = 0; /* abuse to mark usage */
> +
> /*
> * Debugging checks.
> *
> @@ -1936,7 +1942,7 @@ static inline void inc_chains(void)
> */
> if (hlock->read != 2 && hlock->check) {
> if (!check_prev_add(curr, hlock, next,
> - distance, &stack_saved))
> + distance, &trace))
> return 0;
> /*
> * Stop after the first non-trylock entry,
> @@ -1962,6 +1968,9 @@ static inline void inc_chains(void)
> }
> return 1;
> out_bug:
> + if (trace.nr_entries && !trace.skip)
> + return_trace(&trace);
> +
> if (!debug_locks_off_graph_unlock())
> return 0;
>