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

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Message-ID: <Pine.LNX.4.44L0.1003041432320.1305-100000@iolanthe.rowland.org>
Date:	Thu, 4 Mar 2010 14:36:09 -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:

> On Thursday 04 March 2010, Alan Stern wrote:
> > 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.
> 
> OK, so what exactly should trigger the thawing of the writeback thread?
> 
> Should we do that unconditionally at one point or wait for a specific situation
> to happen, and how to recognize that situation in the latter case?

My thought was that del_gendisk would do this unconditionally.  It
would make the task unfreezable and then thaw it, to prevent a race.

This assumes that del_gendisk never gets called in the middle of a
sleep transition unless the device is really gone.  If that's not the
case, Pavel's suggestion (make the resume routine do the device removal
asynchronously) might be better.

Alan Stern

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