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

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Date:	Wed, 12 May 2010 10:50:59 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Matt Reimer <mattjreimer@...il.com>
cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	linux-pm <linux-pm@...ts.linux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Machek <pavel@....cz>
Subject: Re: [linux-pm] Is it supposed to be ok to call del_gendisk while 
 userspace is frozen?

On Tue, 11 May 2010, Matt Reimer wrote:

> >> or has a consensus about how to fix this been
> >> achieved? I'm hitting the same problem and have some time to work on a
> >> fix.
> >
> > Generally, it looks like del_gendisk should thaw writeback threads, but not
> > during suspend, only during resume.
> 
> Thawing the writeback thread only during resume does fix the case
> Maxim originally presented:
> 
> 0. build kernel with CONFIG_MMC_UNSAFE_RESUME
> 1. insert SD card
> 2. suspend
> 3. remove SD card while suspended
> 4. resume from suspend hangs
> 
> But if CONFIG_MMC_UNSAFE_RESUME is not set, the kernel oopses during
> suspend because the MMC device suspend times out:
> 
> mmc0: card e624 removed
> **** DPM device timeout: pxa2xx-mci.0 (pxa2xx-mci)
> kernel BUG at /home/mreimer/sdg/android/android-2.1/kernel/drivers/base/power/main.c:453!
> Unable to handle kernel NULL pointer dereference at virtual address 00000000
> pgd = c0004000
> [00000000] *pgd=00000000
> Internal error: Oops: 817 [#1] PREEMPT
> 
> If I thaw the writeback thread unconditionally in del_gendisk() then
> suspend and resume work as expected for both CONFIG_MMC_UNSAFE_RESUME
> set/not set, even when the card is removed while suspended.
> 
> So what is the proper fix?

I don't see any reason not to let del_gendisk thaw the writeback thread
during suspend.  Since the device is going away anyhow, letting the
thread run shouldn't cause any problems.

Alan Stern

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