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Sender: Tejun Heo <htejun@gmail.com>
Date:   Tue, 22 Feb 2022 08:36:16 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     Bart Van Assche <bvanassche@acm.org>, jgg@ziepe.ca,
        linux-kernel@vger.kernel.org,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Haakon Bugge <haakon.bugge@oracle.com>
Subject: Re: [PATCH v2] workqueue: Warn flush attempt using system-wide
 workqueues
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On Thu, Feb 17, 2022 at 08:22:30PM +0900, Tetsuo Handa wrote:
> @@ -335,6 +335,18 @@ enum {
>  	 */
>  	WQ_POWER_EFFICIENT	= 1 << 7,
>  
> +	/*
> +	 * Since flush operation synchronously waits for completion, flushing
> +	 * system-wide workqueues (e.g. system_wq) or a work on a system-wide
> +	 * workqueue might introduce possibility of deadlock due to unexpected
> +	 * locking dependency.
> +	 *
> +	 * This flag emits warning if flush operation is attempted. Don't set
> +	 * this flag on user-defined workqueues, for destroy_workqueue() will
> +	 * involve flush operation.
> +	 */
> +	WQ_WARN_FLUSH_ATTEMPT   = 1 << 8,

Maybe just __WQ_NO_FLUSH?

>  	__WQ_DRAINING		= 1 << 16, /* internal: workqueue is draining */
>  	__WQ_ORDERED		= 1 << 17, /* internal: workqueue is ordered */
>  	__WQ_LEGACY		= 1 << 18, /* internal: create*_workqueue() */
...
> +static void warn_flush_attempt(struct workqueue_struct *wq)
> +{
> +	static DEFINE_RATELIMIT_STATE(flush_warn_rs, 600 * HZ, 1);
> +
> +
> +	/* Use ratelimit for now in order not to flood warning messages. */
> +	ratelimit_set_flags(&flush_warn_rs, RATELIMIT_MSG_ON_RELEASE);
> +	if (!__ratelimit(&flush_warn_rs))
> +		return;

If you're worried about spamming console while conversion is in progress, we
can just print the immediate (and maybe one more) caller with %pf and
__builtin_return_address() so that it only prints out one line.

Thanks.

-- 
tejun