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 -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
