Re: [PATCH 3/3] kernel/workqueue: Suppress a false positive lockdep complaint

From: "Theodore Y. Ts'o" <tytso@mit.edu>
To: Bart Van Assche <bvanassche@acm.org>
Cc: Tejun Heo <tj@kernel.org>,
	linux-kernel@vger.kernel.org,
	Johannes Berg <johannes.berg@intel.com>,
	Christoph Hellwig <hch@lst.de>, Sagi Grimberg <sagi@grimberg.me>
Subject: Re: [PATCH 3/3] kernel/workqueue: Suppress a false positive lockdep complaint
Date: Thu, 25 Oct 2018 11:40:03 -0400	[thread overview]
Message-ID: <20181025154003.GE4970@thunk.org> (raw)
In-Reply-To: <20181025150540.259281-4-bvanassche@acm.org>

On Thu, Oct 25, 2018 at 08:05:40AM -0700, Bart Van Assche wrote:
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index fc9129d5909e..0ef275fe526c 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -1383,6 +1383,10 @@ static void __queue_work(int cpu, struct workqueue_struct *wq,
>  	if (unlikely(wq->flags & __WQ_DRAINING) &&
>  	    WARN_ON_ONCE(!is_chained_work(wq)))
>  		return;
> +
> +	if (!(wq->flags & __WQ_HAS_BEEN_USED))
> +		wq->flags |= __WQ_HAS_BEEN_USED;
> +
>  retry:
>  	if (req_cpu == WORK_CPU_UNBOUND)
>  		cpu = wq_select_unbound_cpu(raw_smp_processor_id());

I was looking to fix this problem as well, and I thought about doing
this, but I thought wq->mutex had to be taken in order to modify
wq->flags --- and that would destroy the scalability of the
add-to-work-to-queue functions.

We could switch all of wq->flags to use the {test,set,clear}_bits()
family of operations, but that seemed like a very large change.

Tejun seemed to be ok with creating a destroy_workqueue_no_drain()
function and using that instead of destroy_workqueue() for the losers
of the cmpxchg lockless initialization code in sb_init_dio_done_wq()
in fs/direct_io.c.

						- Ted