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Date: Tue, 23 Feb 2016 10:34:09 -0500 (EST)
From: Alan Stern <stern@rowland.harvard.edu>
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To: Peter Chen <hzpeterchen@gmail.com>
cc: tj@kernel.org, <florian@mickler.org>, <linux-usb@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <usb-storage@lists.one-eyed-alien.net>
Subject: Re: Freezable workqueue blocks non-freezable workqueue during the
 system resume process
In-Reply-To: <20160223032056.GB12256@shlinux2.ap.freescale.net>
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On Tue, 23 Feb 2016, Peter Chen wrote:

> Hi Tejun Heo and Florian Mickler,
> 
> I have a question that during the system resume process, the freezable
> workqueue can be thawed if there is a non-freezable workqueue is
> blocked (At uninterruptable state)?
> 
> My case like below, I have a USB OTG (Micro-AB) cable is at USB
> Micro-B port, and there is a USB driver on it, and un-plug this
> cable can wake up system from the suspend. There is a non-freezable
> workqueue ci_otg will be scheduled after disconnecting OTG cable,
> and in its worker ci_otg_work, it will try to disconnect USB drive,
> and flush disk information.

These operations probably are not safe while the system is resuming.  
It might be best to make them wait until the resume is finished.

> But flush disk information is done by
> freezable workqueue writeback, it seeems workqueue writeback is
> never got chance to execute, the workqueue ci_otg is waiting there
> forever, and the system is deadlock.

> Both change workqueue ci_otg as freezable or change workqueue writeback
> as non-freezable can fix this problem.

It sounds like making ci_otg freezable is the easiest solution.

> Please ignore it, the system is locked at driver's resume,
> maybe at scsi or usb driver, so of cos, the freezable processes
> can't be thawed.
 
> > [  555.263177] [<c0043b1c>] (flush_work) from [<c0043fac>] (flush_delayed_work+0x48/0x4c)   
> > [  555.271106]  r8:ed5b5000 r7:c0b38a3c r6:eea439cc r5:eea4372c r4:eea4372c
> > [  555.277958] [<c0043f64>] (flush_delayed_work) from [<c00eae18>] (bdi_unregister+0x84/0xec)
> > [  555.286236]  r4:eea43520 r3:20000153
> > [  555.289885] [<c00ead94>] (bdi_unregister) from [<c02c2154>] (blk_cleanup_queue+0x180/0x29c)
> > [  555.298250]  r5:eea43808 r4:eea43400

You might want to complain to the block-layer people about this.  I 
don't know if anything can be done to fix it.

Or maybe flush_work and flush_delayed_work can be changed to avoid 
blocking if the workqueue is frozen.  Tejun?

Alan Stern