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

[Byungchul Park]

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;
>