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

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Date:	Tue, 23 Feb 2010 10:29:44 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Jens Axboe <jens.axboe@...cle.com>
cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	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 Tue, 23 Feb 2010, Jens Axboe wrote:

> On Tue, Feb 16 2010, Alan Stern wrote:
> > On Mon, 15 Feb 2010, Rafael J. Wysocki wrote:
> > 
> > > On Monday 15 February 2010, Maxim Levitsky wrote:
> > > > On Sat, 2010-02-13 at 15:29 +0200, Maxim Levitsky wrote: 
> > > > > I noticed that currently calling del_gendisk leads to sure deadlock if
> > > > > attemped from .suspend or .resume functions.
> > > 
> > > Well, it shouldn't be called from there, then.
> > 
> > Even if drivers avoid calling it from within suspend methods, they have
> > to be able to call it from within resume methods.  After all, the
> > resume method may find that the disk's device has vanished.
> 
> del_gendisk() needs process context at least, since it'll sleep (not
> just for sync/invalidate, but other parts of the destruction as well).

That's not a problem; suspend and resume run in process context.

> > This is a matter for Jens.  Is the bdi writeback task freezable?  If it
> > is, should it be made unfreezable?
> 
> I'm not a big expect on what tasks should be freezable or not. As it
> stands, the writeback tasks will attempt to freeze and thaw with the
> system. I guess that screws the sync from resume call, since it's not
> running and the sync will wait for it to retrieve and finish that work
> item.
> 
> To the suspend experts - can we safely mark the writeback tasks as
> non-freezable?

The reason for freezing those tasks is to avoid writebacks at random
times during a system sleep transition, when the underlying device may
already be suspended, right?

In principle, a device's writeback task could be unfrozen immediately
after the device is resumed.  In practice this might not solve the
problem, since the del_gendisk() call occurs _within_ the device's
resume routine.  I suppose del_gendisk() could be made responsible for 
unfreezing the writeback task.

The best solution would be to have del_gendisk() avoid waiting for the 
writeback task in cases where the underlying device has been removed.  
I don't know if that is feasible, however.

Alan Stern

P.S.: Jens, given a pointer to a struct gendisk or to a struct
request_queue, is there a good way to tell whether there any dirty
buffers for that device waiting to be written out?  This is for
purposes of runtime power management -- in the initial implementation,
I want to avoid powering-down a block device if it is open or has any
dirty buffers.  In other words, only completely idle devices should be
powered down (a good example would be a card reader with no memory card 
inserted).

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