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Date: Tue, 6 Dec 2016 09:36:51 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Vegard Nossum <vegard.nossum@gmail.com>,
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* Peter Zijlstra <peterz@infradead.org> wrote:

> $ git grep DECLARE_WAIT_QUEUE_HEAD_ONSTACK | wc -l
> 28

This debug facility looks sensible. A couple of minor suggestions:

> --- a/include/linux/wait.h
> +++ b/include/linux/wait.h
> @@ -39,6 +39,9 @@ struct wait_bit_queue {
>  struct __wait_queue_head {
>  	spinlock_t		lock;
>  	struct list_head	task_list;
> +#ifdef CONFIG_DEBUG_WAITQUEUE
> +	int			onstack;
> +#endif

The structure will pack better in the debug-enabled case if 'onstack' is next to 
'lock', as spinlock_t is 4 bytes on many architectures.

> -#define __WAIT_QUEUE_HEAD_INITIALIZER(name) {				\
> +#ifdef CONFIG_DEBUG_WAITQUEUE
> +#define ___WAIT_QUEUE_ONSTACK(onstack)	.onstack = (onstack),
> +#else
> +#define ___WAIT_QUEUE_ONSTACK(onstack)
> +#endif

Please help readers by showing the internal structure of the definition:

#ifdef CONFIG_DEBUG_WAITQUEUE
# define ___WAIT_QUEUE_ONSTACK(onstack)	.onstack = (onstack),
#else
# define ___WAIT_QUEUE_ONSTACK(onstack)
#endif


> +static inline void prepare_debug(wait_queue_head_t *q, wait_queue_t *wait)
> +{
> +#ifdef CONFIG_DEBUG_WAITQUEUE
> +	WARN_ON_ONCE(q->onstack && wait->func == autoremove_wake_function)
> +#endif
> +}

I'd name this debug_waitqueue_check() or such - as the 'prepare' is a bit 
misleadig (we don't prepare debugging, we do the debug check here).

> +config DEBUG_WAITQUEUE
> +	bool "Debug waitqueue"
> +	depends on DEBUG_KERNEL

I'd name it DEBUG_SCHED_WAITQUEUE=y and I'd also make it depend on 
CONFIG_DEBUG_SCHED.

LGTM otherwise!

Thanks,

	Ingo