linux-kernel - Re: [PATCH] MMC: fix hang if card was removed during suspend and unsafe resume was enabled

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Date:	Fri, 05 Feb 2010 10:31:42 +0200
From:	Maxim Levitsky <maximlevitsky@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-mmc@...r.kernel.org, Philip Langdale <philipl@...rt.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Jorg Schummer <ext-jorg.2.schummer@...ia.com>,
	linux-pm <linux-pm@...ts.linux-foundation.org>
Subject: Re: [PATCH] MMC: fix hang if card was removed during suspend and
 unsafe resume was enabled

On Thu, 2010-02-04 at 16:09 -0800, Andrew Morton wrote: 
> On Fri,  5 Feb 2010 01:18:15 +0200 Maxim Levitsky <maximlevitsky@...il.com> wrote:
> 
> > Currently removal of the card leads to del_disk called indirectly by mmc core.
> > This function expects userspace to be running, which isn't when .resume is called
> > 
> > Fix that by removing the code that did that in mmc_resume_host. It is possible
> > because card detection logic will kick it later and remove the card.
> 
> I don't really understand.  The above implies that to trigger this bug,
> one needs to physically remove the card during a resume operation.  ie:
> a human-vs-computer race.  Sounds unlikely?
> 
> So...  exactly what steps does the user need to take to trigger this

Sorry for describing this poorly.
The steps are:

-> Have a kernel with CONFIG_MMC_UNSAFE_RESUME
-> Insert MMC/SD card
-> Suspend/hibernate the system
-> While system is hibernated/suspended pull the card off
-> Resume the system
-> Hang


if CONFIG_MMC_UNSAFE_RESUME is set, mmc core allows the user to
suspend/resume the card normally assuming he won't change the card or
modify it in another system. The former case is actually handled quite
well.

if CONFIG_MMC_UNSAFE_RESUME isn't set, it removes the card during
suspend, and I now think (and will test) that this will still hang the
system this time on suspend.

Maybe we can make del_disk behave well if called with userspace frozen?
After all if user calls it, very likely that hardware is absent thus
there is no point in syncing (which I think triggers the hang)....

Best regards,
Maxim Levitsky

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