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

[Maxim Levitsky <maximlevitsky@gmail.com>]

On Thu, 2010-02-04 at 16:09 -0800, Andrew Morton wrote: 
> On Fri,  5 Feb 2010 01:18:15 +0200 Maxim Levitsky <maximlevitsky@gmail.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