Re: RFC: Bug in generic_forget_inode() ?

[Andrew Morton <akpm@osdl.org>]

Russ Weight <rweight@us.ibm.com> wrote:
>
> On Fri, 2005-03-18 at 14:17 -0800, Andrew Morton wrote:
> > Russ Weight <rweight@us.ibm.com> wrote:
> > >
> > > generic_forget_inode() is eventually called (within the context of
> > >  iput), the inode is placed on the unused list, and the inode_lock is
> > >  dropped.
> > > 
> > >  kswapd calls prune_icache(), locks the inode_lock, and pulls the same
> > >  inode off of the unused list. Upon completion, prune_icache() calls
> > >  dispose_list() for the inodes that it has collected.
> > > 
> > >  generic_forget_inode() calls write_inode_now(), which calls
> > >  __writeback_single_inode() which calls __sync_single_inode().
> > >  __sync_single_inode() panics when attempting to move the inode onto the
> > >  unused list (the last call to list_move). This is due to the poison
> > >  values that were previously loaded into the next and prev list pointers
> > >  by list_del().
> > 
> > It's not clear what the actual bug is here.  When you say that
> > __sync_single_inode() panics over the list pointers, who was it that
> > poisoned them?  dispose_list()?
> > 
> > Certainly isofs_fill_super() could trivially be rewritten to not do the
> > iget()/iput() but we should be sure that that's really the bug.  The inode
> > lifetime management is rather messy, I'm afraid.
> 
> The pointers are poisoned by dispose_list(). The race condition is
> between prune_icache() and generic_forget_inode().
> 
> When I suggested that a change to isofs_fill_super() might be
> considered, I was speculating that isofs_fill_super() might be creating
> an unexpected state by doing something unconventional in its usage of
> iget() and iput(). This is something I had not investigated.
> 
> The problem is more likely in generic_forget_inode(). It releases the
> inode_lock and then locks it again without doing anything to prevent the
> inode from being stolen, and without checking to see if it has been
> stolen. Likewise, write_inode_now() does not do any checks to see if the
> inode has been stolen.
> 

I dunno.  This is really fiddly code and does like to blow up in your face.
I doubt if the problem is in such a well-trodden path as
generic_forget_inode().

Perhaps isofs is breaking the rules by running iget() prior to setting
MS_ACTIVE.  (What rules, you ask?  hah.)