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SubjectRe: mount stuck, khubd blocked
On Thu, 21 Jun 2012, Dave Chinner wrote:

> > > As it is, I think that invalidate_partition() is doing something
> > > somewhat insane for a block device that has been removed - you can't
> > > write to it so fsync_bdev() is useless.
> >
> > That depends. If by "removed" you mean physically disconnected from
> > the computer, then yes. But if "removed" means merely unregistered
> > from the device core then writes can still succeed.
> > invalidate_partition() doesn't know which has happened.
>
> Which means the lower layers probably need to pass that distinction
> up to the invalidation function.

I don't think that information is passed anywhere in the kernel. And
in any case, it's not really important. When a device is unregistered,
the upper layers shouldn't care about the reason why.

> > > And another question - why doesn't having an active filesystem on a
> > > block device (i.e. an active reference to the gendisk) prevent the
> > > block device from being removed from underneath it?
> >
> > References prevent data structures from being deallocated, not from
> > being unregistered (or as James Bottomley likes to call it, "removed
> > from visibility").
>
> Except the unregister path appears to assume that a valid block
> device available when it is unregistered.

It may very well be available during the unregistration procedure.
There's nothing wrong with assuming it is -- if it isn't, I/O attempts
will simply fail.

> That seems to me like
> there is a bad assumption being made in this error handling path...

No; a bad assumption would be if the code assumed the device was
available _after_ the unregistration call had completed.

Alan Stern



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