linux-kernel - Re: [linux-pm] Is it supposed to be ok to call del

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Date:	Wed, 3 Mar 2010 21:48:39 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	Pavel Machek <pavel@....cz>, Jens Axboe <jens.axboe@...cle.com>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	linux-pm <linux-pm@...ts.linux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [linux-pm] Is it supposed to be ok to call del_gendisk while
 userspace is frozen?

On Thu, 4 Mar 2010, Rafael J. Wysocki wrote:

> > Very well.  Then we still need a solution to the original problem:  
> > Devices sometimes need to be unregistered during resume, but
> > del_gendisk() blocks on the writeback thread, which is frozen until
> > after the resume finishes.  How do you suggest this be fixed?
> 
> I thought about thawing the writeback thread earlier in such cases.
> 
> Would that makes sense / is it doable at all?

My thought exactly.  This is the only approach that also solves the 
following race:

	A driver is unloaded at the same time as a suspend starts.

	The writeback thread gets frozen.

	Then before the rmmod thread is frozen, it calls del_gendisk.

Delaying things by means of a workqueue (or the equivalent) might also 
work, but it doesn't seem as safe.  For example, some important 
writebacks might end up getting delayed until too late.

Alan Stern

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