Re: [PATCH RFC] workqueue: remove in_workqueue_context()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Sep 23, 2010 at 02:35:07PM +0200, Tejun Heo wrote:
> Commit a25909a4 (lockdep: Add an in_workqueue_context() lockdep-based
> test function) added in_workqueue_context() but there hasn't been any
> in-kernel user and the lockdep annotation in workqueue is scheduled to
> change.  Remove the unused function.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
> Paul, I was looking through lockdep annotations to see how they can be
> improved and noticed that in_workqueue_context() isn't used by anyone
> and it seems like there hasn't been any in-kernel user either.  Can
> this be removed or do you expect to use it somewhere?

I added it for the drivers/vhost changes.  They were using a
variant of RCU where executing in workqueue context acted as an RCU
read-side critical section and where flushing workqueues acted as
an RCU grace period.  The in_workqueue_context() was then passed to
rcu_dereference_check() to verify correct usage.

But you are right, I don't see it being used any more, though I do still
see the big block comment documenting this in drivers/vhost/vhost.h.

Michael, what are Tejun and I missing here?

							Thanx, Paul

> Thanks.
> 
>  include/linux/workqueue.h |    4 ----
>  kernel/workqueue.c        |   15 ---------------
>  2 files changed, 19 deletions(-)
> 
> diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
> index e33ff4a..f6eaf56 100644
> --- a/include/linux/workqueue.h
> +++ b/include/linux/workqueue.h
> @@ -429,8 +429,4 @@ extern bool freeze_workqueues_busy(void);
>  extern void thaw_workqueues(void);
>  #endif /* CONFIG_FREEZER */
> 
> -#ifdef CONFIG_LOCKDEP
> -int in_workqueue_context(struct workqueue_struct *wq);
> -#endif
> -
>  #endif
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 19e4bc1..c758e78 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -310,21 +310,6 @@ static inline int __next_wq_cpu(int cpu, const struct cpumask *mask,
>  	     (cpu) < WORK_CPU_NONE;					\
>  	     (cpu) = __next_wq_cpu((cpu), cpu_possible_mask, (wq)))
> 
> -#ifdef CONFIG_LOCKDEP
> -/**
> - * in_workqueue_context() - in context of specified workqueue?
> - * @wq: the workqueue of interest
> - *
> - * Checks lockdep state to see if the current task is executing from
> - * within a workqueue item.  This function exists only if lockdep is
> - * enabled.
> - */
> -int in_workqueue_context(struct workqueue_struct *wq)
> -{
> -	return lock_is_held(&wq->lockdep_map);
> -}
> -#endif
> -
>  #ifdef CONFIG_DEBUG_OBJECTS_WORK
> 
>  static struct debug_obj_descr work_debug_descr;