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

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Message-Id: <201003042104.39600.rjw@sisk.pl>
Date:	Thu, 4 Mar 2010 21:04:39 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
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 Thursday 04 March 2010, Alan Stern wrote:
> 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.

Well, I guess we can make it a rule.

Or use a flag that will be set to 'true' during resume (early) and will tell
del_gendisk whether to thaw the writeback thread.

Rafael
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