linux-kernel - Re: mount stuck, khubd blocked

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Date:	Thu, 21 Jun 2012 10:25:02 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Dave Chinner <david@...morbit.com>
cc:	Dima Tisnek <dimaqq@...il.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Jens Axboe <axboe@...nel.dk>,
	USB list <linux-usb@...r.kernel.org>,
	<linux-fsdevel@...r.kernel.org>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: 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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