linux-kernel - Re: [PATCH v2 2/2] lockdep: Remove unnecessary acquisitions wrt workqueue flush

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Date:   Mon, 18 Sep 2017 17:46:22 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     tj@...nel.org, johannes.berg@...el.com, peterz@...radead.org,
        mingo@...nel.org
Cc:     tglx@...utronix.de, oleg@...hat.com, linux-kernel@...r.kernel.org,
        kernel-team@....com
Subject: Re: [PATCH v2 2/2] lockdep: Remove unnecessary acquisitions wrt
 workqueue flush

On Fri, Sep 08, 2017 at 05:46:25PM +0900, Byungchul Park wrote:
> The workqueue added manual acquisitions to catch deadlock cases.
> Now crossrelease was introduced, some of those are redundant, since
> wait_for_completion() already includes the acquisition for itself.
> Removed it.

Now, lock_map_acquire() is already embedded in wait_for_completion(),
manual acquisitions for the same purpose should be removed, since they
would be duplicated. Consider this, please.

> Signed-off-by: Byungchul Park <byungchul.park@....com>
> ---
>  include/linux/workqueue.h |  4 ++--
>  kernel/workqueue.c        | 20 ++++----------------
>  2 files changed, 6 insertions(+), 18 deletions(-)
> 
> diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
> index db6dc9d..1bef13e 100644
> --- a/include/linux/workqueue.h
> +++ b/include/linux/workqueue.h
> @@ -218,7 +218,7 @@ static inline void destroy_delayed_work_on_stack(struct delayed_work *work) { }
>  									\
>  		__init_work((_work), _onstack);				\
>  		(_work)->data = (atomic_long_t) WORK_DATA_INIT();	\
> -		lockdep_init_map(&(_work)->lockdep_map, #_work, &__key, 0); \
> +		lockdep_init_map(&(_work)->lockdep_map, "(complete)"#_work, &__key, 0); \
>  		INIT_LIST_HEAD(&(_work)->entry);			\
>  		(_work)->func = (_func);				\
>  	} while (0)
> @@ -398,7 +398,7 @@ enum {
>  	static struct lock_class_key __key;				\
>  	const char *__lock_name;					\
>  									\
> -	__lock_name = #fmt#args;					\
> +	__lock_name = "(complete)"#fmt#args;				\
>  									\
>  	__alloc_workqueue_key((fmt), (flags), (max_active),		\
>  			      &__key, __lock_name, ##args);		\
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index ab3c0dc..72f68b1 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -2497,15 +2497,8 @@ static void insert_wq_barrier(struct pool_workqueue *pwq,
>  	INIT_WORK_ONSTACK(&barr->work, wq_barrier_func);
>  	__set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&barr->work));
>  
> -	/*
> -	 * Explicitly init the crosslock for wq_barrier::done, make its lock
> -	 * key a subkey of the corresponding work. As a result we won't
> -	 * build a dependency between wq_barrier::done and unrelated work.
> -	 */
> -	lockdep_init_map_crosslock((struct lockdep_map *)&barr->done.map,
> -				   "(complete)wq_barr::done",
> -				   target->lockdep_map.key, 1);
> -	__init_completion(&barr->done);
> +	init_completion_with_map(&barr->done, &target->lockdep_map);
> +
>  	barr->task = current;
>  
>  	/*
> @@ -2611,16 +2604,14 @@ void flush_workqueue(struct workqueue_struct *wq)
>  	struct wq_flusher this_flusher = {
>  		.list = LIST_HEAD_INIT(this_flusher.list),
>  		.flush_color = -1,
> -		.done = COMPLETION_INITIALIZER_ONSTACK(this_flusher.done),
>  	};
>  	int next_color;
>  
> +	init_completion_with_map(&this_flusher.done, &wq->lockdep_map);
> +
>  	if (WARN_ON(!wq_online))
>  		return;
>  
> -	lock_map_acquire(&wq->lockdep_map);
> -	lock_map_release(&wq->lockdep_map);
> -
>  	mutex_lock(&wq->mutex);
>  
>  	/*
> @@ -2883,9 +2874,6 @@ bool flush_work(struct work_struct *work)
>  	if (WARN_ON(!wq_online))
>  		return false;
>  
> -	lock_map_acquire(&work->lockdep_map);
> -	lock_map_release(&work->lockdep_map);
> -
>  	if (start_flush_work(work, &barr)) {
>  		wait_for_completion(&barr.done);
>  		destroy_work_on_stack(&barr.work);
> -- 
> 1.9.1