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From: Alan Stern on 12 May 2010 11:00
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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From: Alan Stern on 13 May 2010 18:00
On Thu, 13 May 2010, Matt Reimer wrote:

> So how does the attached patch look?
>
> Matt
>
>
> From 20d8340471eb05aa54af1349f4ddccecd9c230c6 Mon Sep 17 00:00:00 2001
> From: Matt Reimer <mreimer(a)sdgsystems.com>
> Date: Thu, 13 May 2010 14:36:54 -0700
> Subject: [PATCH] fs: prevent hang on suspend/resume when MMC/SD card present
>
> Devices can come and go from the MMC/SD bus during suspend or resume,
> when the writeback thread is frozen, resulting in a hang. So thaw the
> writeback thread in del_gendisk() to prevent the hang.

I don't see anything wrong with the patch itself, but I dislike the
description. Devices can come and go from any hotpluggable bus, not
just MMC/SD. That just happens to be the first place the problem was
observed.

Alan Stern

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From: Alan Stern on 14 May 2010 22:40
On Thu, 13 May 2010, Matt Reimer wrote:

> > I don't see anything wrong with the patch itself, but I dislike the
> > description. �Devices can come and go from any hotpluggable bus, not
> > just MMC/SD. �That just happens to be the first place the problem was
> > observed.
>
> Good point. How about this?
>
> Matt
>
> From 813bd223e5a2fa577b9e64ddf12654a93d0aab8b Mon Sep 17 00:00:00 2001
> From: Matt Reimer <mreimer(a)sdgsystems.com>
> Date: Thu, 13 May 2010 14:36:54 -0700
> Subject: [PATCH] fs: prevent hang on suspend/resume when MMC/SD card present
>
> Devices can come and go bus during suspend or resume, when the
> writeback thread is frozen, resulting in a hang. Prevent the hang
> by thawing the writeback thread in del_gendisk().

I would have said "the block layer's writeback thread", but this is
okay.

Alan Stern

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