Re: [-mm patch] Fix inotify umount hangs.

[Anton Altaparmakov]

On Tue, 5 Jul 2005, Robert Love wrote:
> On Mon, 2005-07-04 at 20:28 +0100, Anton Altaparmakov wrote:
> > The below patch against 2.6.13-rc1-mm1 fixes the umount hangs caused by 
> > inotify.
> 
> Thank you, very much, Anton, for hacking on this over the weekend.

You are welcome.  (-:

> It's definitely not the prettiest thing, but there may be no easier
> approach.  One thing, the messy code is working around the list
> changing, doesn't invalidate_inodes() have the same problem?  If so, it
> solves it differently.

It does.  It first goes over the i_sb_list and anything it finds that it 
is interested in (i.e. all inodes with zero i_count), it moves the inode 
(i_list) over to a private list (this is done in invalidate_list()).  
Then, when it is finished accessing the i_sb_list, and all inodes of 
interest (zero i_count) are on the private list, dispose_list() is called, 
and all inodes on the private list are exterminated.  This obviously no 
longer uses i_sb_list so it does not matter that that is changing now.

> I'm also curious if the I_WILL_FREE or i_count check fixed the bug.  I
> suspect the other fix did, but we probably still want this.  Or at least
> the I_WILL_FREE check.

The i_count check is at least sensible if not required (not sure) 
otherwise you do iget() on inode with zero i_count then waste your time 
looking for watches (which can't be there or i_count would not be zero), 
and then iput() kills off the inode and throws it out of the icache.  This 
would be done in invalidate_inodes() anyway, no need for you to do it 
first.  Even if this is not a required check it saves some cpu cycles.

The I_WILL_FREE is definitely required otherwise you will catch inodes 
that will suddenly become I_FREEING and then I_CLEAR under your feet once 
you drop the inode_lock and we know that is a Bad Thing (TM) as it causes 
the BUG_ON(i_state == I_CLEAR) in iput() to trigger that was reported 
before (when I got drawn into inotify in the first place).

Note that if you want to be really thorough you could wait on the inode 
to be destroyed for good:

if (inode->i_state & (I_FREEING|I_CLEAR|I_WILL_FREE))
	__wait_on_freeing_inode(inode);

And then re-check in the i_sb_list if the inode is still there (e.g. via 
prev member of next_i->i_sb_list which will no longer be "inode" if the 
inode has been evicted).  If the inode is still there someone 
"reactivated" it while you were waiting and you need to redo the 
i_count and i_state checks and deal with the inode as appropriate.

However given this is umount we are talking about there doesn't seem to be 
much point in being that thorough.

I am not familiar enough with i_notify but _if_ it is possible for a user 
to get a watch on an inode which is I_FREEING|I_CLEAR|I_WILLFREE then you 
have to do the waiting otherwise you will miss that watch with I don't 
know what results but probably not good ones...

Best regards,

	Anton
-- 
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer / IRC: #ntfs on irc.freenode.net
WWW: http://linux-ntfs.sf.net/ & http://www-stu.christs.cam.ac.uk/~aia21/
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