linux-kernel - Re: [RFC] deadlock with flush

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Message-ID: <1561366612.2846.10.camel@suse.com>
Date:   Mon, 24 Jun 2019 10:56:52 +0200
From:   Oliver Neukum <oneukum@...e.com>
To:     Tejun Heo <tj@...nel.org>, Alan Stern <stern@...land.harvard.edu>
Cc:     Kernel development list <linux-kernel@...r.kernel.org>,
        USB list <linux-usb@...r.kernel.org>
Subject: Re: [RFC] deadlock with flush_work() in UAS

Am Donnerstag, den 20.06.2019, 07:10 -0700 schrieb Tejun Heo:
> Hello,
> 
> On Tue, Jun 18, 2019 at 11:59:39AM -0400, Alan Stern wrote:
> > > > Even if you disagree, perhaps we should have a global workqueue with a
> > > > permanently set noio flag.  It could be shared among multiple drivers
> > > > such as uas and the hub driver for purposes like this.  (In fact, the 
> > > > hub driver already has its own dedicated workqueue.)
> > > 
> > > That is a good idea. But does UAS need WQ_MEM_RECLAIM?
> > 
> > These are good questions, and I don't have the answers.  Perhaps Tejun 
> > or someone else on LKML can help.
> 
> Any device which may host a filesystem or swap needs to use
> WQ_MEM_RECLAIM workqueues on anything which may be used during normal
> IOs including e.g. error handling which may be invoked.  One
> WQ_MEM_RECLAIM workqueue guarantees one level of concurrency for all
> its tasks regardless of memory situation, so as long as there's no
> interdependence between work items, the workqueue can be shared.

Ouch.

Alan, in that case anything doing a reset, suspend or resume needs
to use WQ_MEM_RECLAIM, it looks to me. What do we do?

	Regards
		Oliver