[PATCH] lockdep: Remove unnecessary acquisitions wrt workqueue flush

[PATCH] lockdep: Remove unnecessary acquisitions wrt workqueue flush

[Byungchul Park]

Workqueue added manual acquisitions to catch deadlock cases. Now
crossrelease was introduced, some of those are redundant because
crossrelease-enabled wait_for_completeion() also does it. Removed it.

Signed-off-by: Byungchul Park <byungchul.park@lge.com>
---
 include/linux/workqueue.h |  4 ++--
 kernel/workqueue.c        | 16 +++++++---------
 2 files changed, 9 insertions(+), 11 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..ea4e9b6 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2503,8 +2503,8 @@ static void insert_wq_barrier(struct pool_workqueue *pwq,
 	 * 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);
+				   target->lockdep_map.name,
+				   target->lockdep_map.key, 0);
 	__init_completion(&barr->done);
 	barr->task = current;
 
@@ -2611,16 +2611,17 @@ 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;
 
+	lockdep_init_map_crosslock((struct lockdep_map *)&this_flusher.done.map,
+				   wq->lockdep_map.name,
+				   wq->lockdep_map.key, 0);
+	__init_completion(&this_flusher.done);
+
 	if (WARN_ON(!wq_online))
 		return;
 
-	lock_map_acquire(&wq->lockdep_map);
-	lock_map_release(&wq->lockdep_map);
-
 	mutex_lock(&wq->mutex);
 
 	/*
@@ -2883,9 +2884,6 @@ 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

Re: [PATCH] lockdep: Remove unnecessary acquisitions wrt workqueue flush

[Peter Zijlstra]

On Thu, Sep 07, 2017 at 09:33:16AM +0900, Byungchul Park wrote:
> Workqueue added manual acquisitions to catch deadlock cases. Now
> crossrelease was introduced, some of those are redundant because
> crossrelease-enabled wait_for_completeion() also does it. Removed it.

This does not explain how it doesn't become the regular timing dependent
stuff cross-release normally is.

Re: [PATCH] lockdep: Remove unnecessary acquisitions wrt workqueue flush

[Byungchul Park]

On Thu, Sep 7, 2017 at 6:41 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Thu, Sep 07, 2017 at 09:33:16AM +0900, Byungchul Park wrote:
>> Workqueue added manual acquisitions to catch deadlock cases. Now
>> crossrelease was introduced, some of those are redundant because
>> crossrelease-enabled wait_for_completeion() also does it. Removed it.
>
> This does not explain how it doesn't become the regular timing dependent
> stuff cross-release normally is.

Yes, I will add more explanation for it at the next spin.

But, let me explain it a bit here to help you get it, since it's very
simple. This has nothing to do with 'might' thing I introduced
previously. For 'might' thing, I will start it after getting your response.

Here, I just removed acquire() because wait_for_completion()
already includes the acquire() by LOCKDEP_COMPLETIONS.
That's all.

-- 
Thanks,
Byungchul

Re: [PATCH] lockdep: Remove unnecessary acquisitions wrt workqueue flush

[Peter Zijlstra]

On Thu, Sep 07, 2017 at 08:37:08PM +0900, Byungchul Park wrote:
> On Thu, Sep 7, 2017 at 6:41 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Thu, Sep 07, 2017 at 09:33:16AM +0900, Byungchul Park wrote:
> >> Workqueue added manual acquisitions to catch deadlock cases. Now
> >> crossrelease was introduced, some of those are redundant because
> >> crossrelease-enabled wait_for_completeion() also does it. Removed it.
> >
> > This does not explain how it doesn't become the regular timing dependent
> > stuff cross-release normally is.
> 
> Yes, I will add more explanation for it at the next spin.
> 
> But, let me explain it a bit here to help you get it, since it's very
> simple. This has nothing to do with 'might' thing I introduced
> previously. For 'might' thing, I will start it after getting your response.
> 
> Here, I just removed acquire() because wait_for_completion()
> already includes the acquire() by LOCKDEP_COMPLETIONS.
> That's all.

Ah I see, but it does so using some fairly nasty completion init stuff.
The old code was far easier to read.

Re: [PATCH] lockdep: Remove unnecessary acquisitions wrt workqueue flush

[Byungchul Park]

On Fri, Sep 8, 2017 at 12:21 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Thu, Sep 07, 2017 at 08:37:08PM +0900, Byungchul Park wrote:
>> On Thu, Sep 7, 2017 at 6:41 PM, Peter Zijlstra <peterz@infradead.org> wrote:
>> > On Thu, Sep 07, 2017 at 09:33:16AM +0900, Byungchul Park wrote:
>> >> Workqueue added manual acquisitions to catch deadlock cases. Now
>> >> crossrelease was introduced, some of those are redundant because
>> >> crossrelease-enabled wait_for_completeion() also does it. Removed it.
>> >
>> > This does not explain how it doesn't become the regular timing dependent
>> > stuff cross-release normally is.
>>
>> Yes, I will add more explanation for it at the next spin.
>>
>> But, let me explain it a bit here to help you get it, since it's very
>> simple. This has nothing to do with 'might' thing I introduced
>> previously. For 'might' thing, I will start it after getting your response.
>>
>> Here, I just removed acquire() because wait_for_completion()
>> already includes the acquire() by LOCKDEP_COMPLETIONS.
>> That's all.
>
> Ah I see, but it does so using some fairly nasty completion init stuff.
> The old code was far easier to read.

Agree.

But, old code has redundant acquisitions, moreover uses sub-class
to avoid false detection, even though they are the same class.

I will try to enhance readability of completion init, and fix the incorrect
use at the next spin.

-- 
Thanks,
Byungchul