linux-kernel - Re: [PATCH] fs: Fix mod_timer crash when removing USB sticks

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Date:	Sat, 17 Mar 2012 10:21:43 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Theodore Tso <tytso@...gle.com>
cc:	Greg KH <greg@...ah.com>, Paul Taysom <taysom@...gle.com>,
	Paul Taysom <taysom@...omium.org>,
	Mandeep Baines <msb@...omium.org>,
	Jens Axboe <axboe@...nel.dk>, Andrew Morton <akpm@...gle.com>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	<linux-fsdevel@...r.kernel.org>, <stable@...nel.org>
Subject: Re: [PATCH] fs: Fix mod_timer crash when removing USB sticks

On Fri, 16 Mar 2012, Theodore Tso wrote:

> I thought another fix at the USB layer also went in that attempted to
> fix this problem for 3.2, and so with two separate band-aid patches, I
> think we had thought the problem had been addressed.

I can't recall any USB fix like that.  The only thing I remember is 
your change to ext4 (leaving the problem still present in ext3).

> The real problem is that all of the patches which I've seen to date
> are band-aids, in that we aren't properly sending a "device as
> disappeared" notification to the file system layer, but instead we are
> trying to keep enough of the pointers valid (while also freeing other
> data structures), such that the file system can blindly write into a
> partially dismantled block device, and hopefully not oops.

That's not a band-aid approach; it's the way reference counting is 
_intended_ to work.  The whole idea of refcounting is that instead of 
synchronizing every single operation, you keep data structures around 
so long as anyone might be using them.

> Some have argued that my suggested approach of having an explicit
> super_ops revoke() function, which tells the file system that the
> block device is gone, etc., isn't necessary because this can be solved
> in userspace somehow.  Personally I think that's nuts, since we'll
> continue to play whack-a-mole, but I haven't had time to work up
> patches addressing this --- since this is really only a problem for
> naive users who pull USB sticks without unmounting them first (and so
> it never happens to me :-), and I've got a lot of other fish to
> try.....

I suspect Paul's patch is the right thing to do.  It might even make
the ext4 fix unnecessary, although I don't understand the details well
enough to verify it.  Maybe Paul can check -- the commit I'm referring
to is 7c2e70879fc0949b4220ee61b7c4553f6976a94d (ext4: add ext4-specific
kludge to avoid an oops after the disk disappears).

Alan Stern

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