Re: [linux-pm] [PATCH] Fix the outstanding issue with hangs on insert/removal of mmc cards

[Maxim Levitsky <maximlevitsky@gmail.com>]

On Fri, 2010-06-11 at 17:00 -0400, Alan Stern wrote: 
> On Fri, 11 Jun 2010, Maxim Levitsky wrote:
> 
> > Hi,
> > 
> > After thinking a lot about how to fix properly the hangs caused by
> > insert/removal of mmc card during suspend/resume, and default behavior
> > of not trusting the card persistence over suspend, I finally come to
> > conclusion that changing the del_gendisk is wrong.
> > 
> > First of all there are 2 types of removal possible. First one happens
> > when system detects that some device is gone. At that point there is
> > really no point in syncing it.
> > 
> > The other type of removal is controlled removal, usually on user
> > request. Surly we must sync the device of this request.
> > This type of removal _shouldn't_ happen during suspend/resume
> > transaction. The only case when it does is today to protect against user
> > carelessness of switching the cards during suspend.
> 
> There are other pathological cases which can cause it to happen, but 
> they are pretty unlikely.
> 
> > I think that it is just wrong to sync the device in suspend/resume time.
> > At that time userspace is frozen, but also its not known which drivers
> > are still running. They might even suspend asynchronously...
> > So, such cases should be moved to pm-notifier, thing that my patch does
> > for mmc.
> > Other users should be fixed as well.
> > 
> > We can, in addition to that, add a temporary hack to del_gendisk with
> > loud WARN_ON though.
> > 
> > If card is really removed during suspend, then we can just introduce
> > del_gendisk_dead or something like that which will be safe to call
> > during suspend.
> > 
> > I didn't do that but rather I made the card detection thread freezeable,
> > thus eliminated the whole problem.
> > If you remove the card during suspend, system will notice at end of
> > resume.
> 
> I don't know why the mmc subsystem works differently from USB.  In USB,
> the equivalent of UNSAFE_RESUME is a per-device flag that can be
> controlled via sysfs (see Documentation/usb/persist.txt).  And it
> almost always defaults to ON, i.e., the kernel assumes that if a device
> is present before suspend and after resume it is the same device --
> although some checking is done to try to verify this (the descriptors
> have to remain the same).  We started out being more cautious (the
> default was OFF), but Linus complained about it being _too_ cautious.
I will be very happy to see the description and default value of 
MMC_UNSAFE_RESUME changed.

This patch fixes both cases.

> 
> And like you have done here, in USB the kernel thread that handles
> registering and unregistering devices is freezable, so things never get
> added or removed at an unsafe time.
Very nice!

Best regards,
Maxim Levitsky