Re: [PATCH] lockdep: fix stack trace caching logic

[Peter Hurley]

On 02/04/2016 05:40 AM, Dmitry Vyukov wrote:
> check_prev_add() caches saved stack trace in static trace variable
> to avoid duplicate save_trace() calls in dependencies involving trylocks.
> But that caching logic contains a bug.

> We may not save trace on first iteration due to early return from check_prev_add().

This commit log should identify the role test instrumentation plays in triggering
this bug: is it a recursive read lock dependency injected between existing lock
dependencies? What test component triggered this?


> Then on the second iteration when we actually need the trace we don't save it
> because we think that we've already saved it.
> 
> Let check_prev_add() itself control when stack is saved.
> 
> There is another bug. Trace variable is protected by graph lock.
> But we can temporary release graph lock during printing.
> 
> Fix this by invalidating cached stack trace when we release graph lock.
> 
> Signed-off-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> ---
>  kernel/locking/lockdep.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 60ace56..c7710e4 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -1822,7 +1822,7 @@ check_deadlock(struct task_struct *curr, struct held_lock *next,
>   */
>  static int
>  check_prev_add(struct task_struct *curr, struct held_lock *prev,
> -	       struct held_lock *next, int distance, int trylock_loop)
> +	       struct held_lock *next, int distance, int *stack_saved)
>  {
>  	struct lock_list *entry;
>  	int ret;
> @@ -1883,8 +1883,11 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
>  		}
>  	}
>  
> -	if (!trylock_loop && !save_trace(&trace))
> -		return 0;
> +	if (!*stack_saved) {
> +		if (!save_trace(&trace))
> +			return 0;
> +		*stack_saved = 1;
> +	}
>  
>  	/*
>  	 * Ok, all validations passed, add the new lock
> @@ -1907,6 +1910,8 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
>  	 * 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));
> @@ -1929,7 +1934,7 @@ static int
>  check_prevs_add(struct task_struct *curr, struct held_lock *next)
>  {
>  	int depth = curr->lockdep_depth;
> -	int trylock_loop = 0;
> +	int stack_saved = 0;
>  	struct held_lock *hlock;
>  
>  	/*
> @@ -1956,7 +1961,7 @@ check_prevs_add(struct task_struct *curr, struct held_lock *next)
>  		 */
>  		if (hlock->read != 2 && hlock->check) {
>  			if (!check_prev_add(curr, hlock, next,
> -						distance, trylock_loop))
> +						distance, &stack_saved))
>  				return 0;
>  			/*
>  			 * Stop after the first non-trylock entry,
> @@ -1979,7 +1984,6 @@ check_prevs_add(struct task_struct *curr, struct held_lock *next)
>  		if (curr->held_locks[depth].irq_context !=
>  				curr->held_locks[depth-1].irq_context)
>  			break;
> -		trylock_loop = 1;
>  	}
>  	return 1;
>  out_bug:
>