Subject: Re: inode_unused list corruption in 2.4.26 - spin_lock problem?
From: Trond Myklebust <trond.myklebust@fys.uio.no>
To: Marcelo Tosatti <marcelo.tosatti@cyclades.com>
Cc: Chris Caputo <ccaputo@alt.net>, linux-kernel@vger.kernel.org,
       David Woodhouse <dwmw2@infradead.org>, riel@redhat.com
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P? su , 20/06/2004 klokka 20:45, skreiv Marcelo Tosatti:
> Lets see if I get this right, while we drop the lock in iput to call 
> write_inode_now() an iget happens, possibly from write_inode_now itself 
> (sync_one->__iget) causing the inode->i_list to be added to to inode_in_use. 
> But then the call returns, locks inode_lock, decreases inodes_stat.nr_unused--
> and deletes the inode from the inode_in_use and adds to inode_unused. 
> 
> AFAICS its an inode with i_count==1 in the unused list, which does not
> mean "list corruption", right? Am I missing something here?

Yes. Please don't forget that the inode is still hashed and is not yet
marked as FREEING: find_inode() can grab it on behalf of some other
process as soon as we drop that spinlock inside iput(). Then we have the
calls to clear_inode() + destroy_inode() just a few lines further down.
;-)

If the above scenario ever does occur, it will cause random Oopses for
third party processes. Since we do not see this too often, my guess is
that the write_inode_now() path must be very rarely (or never?) called.

> If you are indeed right all 2.4.x versions contain this bug.

...and all 2.6.x versions...

I'm not saying this is the same problem that Chris is seeing, but I am
failing to see how iput() is safe as it stands right now. Please
enlighten me if I'm missing something.

Cheers,
  Trond
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